[DebugInfo] -fdebug-prefix-map: handle '#line "file"' for asm source

`getContext().setMCLineTableRootFile` (from D62074) sets `RootFile.Name` to
`FirstCppHashFilename`. `RootFile.Name` is not processed by -fdebug-prefix-map
and will go to DW_TAG_compile_unit's DT_AT_name and DW_TAG_label's
DW_AT_decl_file. Remap `RootFile.Name`.

Fix another issue reported by https://github.com/llvm/llvm-project/issues/56609

Reviewed By: #debug-info, dblaikie, raj.khem

Differential Revision: https://reviews.llvm.org/D131848

(cherry picked from commit d797c2ff)