py-clang: enable py314 and clang-21, update clang-20 src

[percontation]

Description

Enable new major versions of python and clang in py-clang.

Type(s)

• enhancement
• update

Tested on

macOS 15.7.1 24G231 arm64
Xcode 26.0.1 17A400

I only tested py314-clang +clang21, py39-clang +clang21, and py312-clang +clang20, not the whole matrix.

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@eborisch for port py-clang.

[percontation]

I opted not to delete the +clang19 variant right now—even though the port file stipulates only keeping the latest two major clang versions—because clang19 was still the current version of clang less than 8 months ago.

I also opted not to bump the portfile version number, even though this theoretically affects current py*-clang +clang20 installs by updating the clang-20 source tarball. This is because the bindings/python files did not change between these two versions of clang-20.