Pin PFA main branch to clang-15

[tpatki]

After an extensive discussion, @slabasan and I have decided that we will pin the main branch of PFA to clang v15.
This will include only the NewPassManager, and the LegacyPassManager will be removed from the main branch.
This will also help clean up our code greatly, as we will remove any Clang version related checks from our CMake files.

(Note: This will automatically support clang v16.)