[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:

@joker-eph After this cmake change msvc bullds have crawled to a halt - typically building on only a couple of cores at a time and with no build distribution at all :( 

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