Test clang-uml with latest 18.1.0 release

[bkryza]

Official release of the LLVM 18 major release has been published so we need to check if clang-uml works with this version properly.

[bkryza]

Note to troubleshooting: When running clang-uml on code using C++20 modules, the LLVM version used to build the project must be compatible with the LLVM version linked to clang-uml, otherwise you'll get error like this:

fatal error: malformed or corrupted AST file: 'malformed block record in AST file'

or like this:

error: PCH file uses an older PCH format that is no longer supported

In particular versions 17 and 18 of LLVM are not compatible in this regard.

Otherwise clang-uml now works with LLVM 18 fine.