tools/clang+llvm/14.0, tools/clang+llvm/15.0: Fix unavailable packages for Clang 14 and 15

Closes #234 (closed)

Changelog entry: using clang+llvm archives from github

Both modules could be simplified drastically. There is only one archive that needs to be downloaded. In addition, the LD_LIBRARY_PATH environment variable does not need to be changed. We had some issues with this in the past, so this is quite beneficial.

Note:

as tar.xz archives cannot be extracted into a dedicated folder, I had to adjust all variables. (Previously, it was extracted into the 'install' folder, now it is e.g. clang+llvm-15.0.6-x86_64-linux-gnu-ubuntu-18.04