[llvm] Strip the full path from __FILE__ in the LDBG macro and keep only the filename (PR #150677)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

@joker-eph I think this completely breaks clang module build since a different macro for each source file means the modules are never shared anymore. This causes https://green.lab.llvm.org/job/clang-stage2-Rthinlto/ running out of space (and really really slow).

Also, why we don't use `__FILE_NAME__` instead? That is a clang extension but we could check and use that before passing the __SHORT_FILE__ macro.

https://github.com/llvm/llvm-project/pull/150677