[clang] [DebugInfo][CodeView] Emit path prefix substitution for COFF object file name. (PR #205729)

[Reid Kleckner via cfe-commits]

https://github.com/rnk requested changes to this pull request.

Code looks good, but please add a test case to ensure we cover this path.

https://github.com/llvm/llvm-project/pull/205729