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

[Jon Roelofs via llvm-commits]

jroelofs wrote:

I reverted this in a15b629527a975ec592c95d69d04ef3537915d1d to get https://green.lab.llvm.org/job/clang-stage2-Rthinlto/ back to green (disabled currently, as it was trashing disk space on all the nodes in the label).

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