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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Its msbuild builds that seem to be affected, not ninja+msvc which is what I think all the buildbots use. I've deleted and rebuilt the build directories and they are still affected (and I ended up bisecting to find this so was regenerating a lot in the process), seen on both Debug and RelWithDebInfo builds so far.

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