This error is clear as crystal and confirmed by macports website: llvm-3.0 is not supported on Mavericks.
Luckily there is a solution. Read on ;)

The error:

flmbp:~ root# [color=green]port -v upgrade outdated[/color]
--->  Computing dependencies for llvm-3.0.
--->  Fetching distfiles for llvm-3.0
[color=red]Error: llvm-3.0 is not supported on Mavericks or later.
Error: org.macports.fetch for port llvm-3.0 returned: unsupported platform[/color]
Warning: targets not executed for llvm-3.0: org.macports.destroot org.macports.fetch org.macports.checksum org.macports.extract org.macports.patch org.macports.configure org.macports.build
Please see the log file for port llvm-3.0 for details:
    /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_ports_lang_llvm-3.0/llvm-3.0/main.log
Error: Unable to upgrade port: 1
To report a bug, follow the instructions in the guide:
    http://guide.macports.org/#project.tickets

The solution:

1. Install llvm-3.3 which is compatible with Mavericks:

flmbp:~ root# [color=green]port -v install llvm-3.3[/color]
--->  Computing dependencies for llvm-3.3.
--->  Fetching archive for llvm-3.3
--->  llvm-3.3-3.3_1.darwin_13.x86_64.tbz2 doesn't seem to exist in /opt/local/var/macports/incoming/verified
--->  Attempting to fetch llvm-3.3-3.3_1.darwin_13.x86_64.tbz2 from http://lil.fr.packages.macports.org/llvm-3.3
--->  Attempting to fetch llvm-3.3-3.3_1.darwin_13.x86_64.tbz2 from http://mse.uk.packages.macports.org/sites/packages.macports.org/llvm-3.3
--->  Attempting to fetch llvm-3.3-3.3_1.darwin_13.x86_64.tbz2 from http://packages.macports.org/llvm-3.3
--->  Fetching distfiles for llvm-3.3
--->  llvm-3.3.src.tar.gz doesn't seem to exist in /opt/local/var/macports/distfiles/llvm
--->  Attempting to fetch llvm-3.3.src.tar.gz from http://lil.fr.distfiles.macports.org/llvm
--->  Verifying checksums for llvm-3.3
--->  Checksumming llvm-3.3.src.tar.gz
--->  Extracting llvm-3.3
--->  Extracting llvm-3.3.src.tar.gz
--->  Applying patches to llvm-3.3
--->  Applying install_target.patch
--->  Applying tiger.patch
--->  Applying tiger-shlib.patch
--->  Configuring llvm-3.3
--->  Cleaning llvm-3.3
--->  Removing work directory for llvm-3.3
--->  Updating database of binaries: 100.0%
--->  Scanning binaries for linking errors: 100.0%
--->  No broken files found.

2. Install ld64 with variant llvm33 (it could be still linked to llvm30):

flmbp:~ root# [color=green] port -v install ld64 +llvm33[/color]
--->  Computing dependencies for ld64.
--->  Fetching archive for ld64
--->  ld64-136_2+llvm33.darwin_13.x86_64.tbz2 doesn't seem to exist in /opt/local/var/macports/incoming/verified
--->  Attempting to fetch ld64-136_2+llvm33.darwin_13.x86_64.tbz2 from http://lil.fr.packages.macports.org/ld64
--->  Attempting to fetch ld64-136_2+llvm33.darwin_13.x86_64.tbz2 from http://mse.uk.packages.macports.org/sites/packages.macports.org/ld64
--->  Attempting to fetch ld64-136_2+llvm33.darwin_13.x86_64.tbz2 from http://packages.macports.org/ld64
--->  Fetching distfiles for ld64
--->  Verifying checksums for ld64
--->  Checksumming ld64-136.tar.gz
--->  Extracting ld64
--->  Extracting ld64-136.tar.gz
--->  Applying patches to ld64
--->  Applying ld64-version.patch
--->  Applying ld64-133-no-CrashReporterClient.h.patch
--->  Applying ld64-134-missing-include.patch
--->  Applying ld64-136-i386-badAddress.patch
--->  Configuring ld64
--->  Building ld64
--->  Cleaning ld64
--->  Removing work directory for ld64
--->  Updating database of binaries: 100.0%
--->  Scanning binaries for linking errors: 100.0%
--->  No broken files found.

3. Install cctools also with variant llvm33:

flmbp:~ root# [color=green] port -v install cctools +llvm33[/color]
--->  Computing dependencies for cctools.
--->  Fetching archive for cctools
--->  cctools-839_2+llvm33.darwin_13.x86_64.tbz2 doesn't seem to exist in /opt/local/var/macports/incoming/verified
--->  Attempting to fetch cctools-839_2+llvm33.darwin_13.x86_64.tbz2 from http://lil.fr.packages.macports.org/cctools
--->  Attempting to fetch cctools-839_2+llvm33.darwin_13.x86_64.tbz2 from http://mse.uk.packages.macports.org/sites/packages.macports.org/cctools
--->  Attempting to fetch cctools-839_2+llvm33.darwin_13.x86_64.tbz2 from http://packages.macports.org/cctools
--->  Fetching distfiles for cctools
--->  Verifying checksums for cctools
--->  Checksumming cctools-839.tar.gz
--->  Extracting cctools
--->  Extracting cctools-839.tar.gz
--->  Applying patches to cctools
--->  Applying cctools-829-lto.patch
--->  Applying PR-37520.patch
--->  Applying cctools-839-static-dis_info.patch
--->  Configuring cctools
--->  Building cctools
--->  Cleaning cctools
--->  Removing work directory for cctools
--->  Updating database of binaries: 100.0%
--->  Scanning binaries for linking errors: 100.0%
--->  No broken files found.

Now port -v upgrade outdated should work. Mine is in progress. I will let you know if everything goes well... or not.

UPDATE:

So after hours of compiling, I got another error regarding llvm-3.0:

Do not worry, solution after the error log.

--->  Cleaning links
--->  Removing work directory for links
--->  Computing dependencies for llvm-3.0.
--->  Fetching distfiles for llvm-3.0
Error: llvm-3.0 is not supported on Mavericks or later.
Error: org.macports.fetch for port llvm-3.0 returned: unsupported platform
Warning: targets not executed for llvm-3.0: org.macports.destroot org.macports.fetch org.macports.checksum org.macports.extract org.macports.patch org.macports.configure org.macports.build
Please see the log file for port llvm-3.0 for details:
    /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_ports_lang_llvm-3.0/llvm-3.0/main.log
Error: Unable to upgrade port: 1
To report a bug, follow the instructions in the guide:
    http://guide.macports.org/#project.tickets

Just uninstall llvm-3.0:

florians-MacBook-Pro:~ root# port -v uninstall llvm-3.0
--->  Deactivating llvm-3.0 @3.0_12
--->  Cleaning llvm-3.0
--->  Removing work directory for llvm-3.0
--->  Uninstalling llvm-3.0 @3.0_12
--->  Cleaning llvm-3.0
--->  Removing work directory for llvm-3.0

Now you can run again port -v upgrade outdated.

And finally... SUCCESS!

--->  Updating database of binaries: 100.0%
--->  Scanning binaries for linking errors: 100.0%
--->  No broken files found.

PS: On MacPorts website (https://trac.macports.org/wiki/Migration) the recommendation is to reinstall MacPorts after upgrading to a new major OS version or CPU architecture. You can do that too if everything else fails (including the above example).