[MCDwarf] Respect -fdebug-prefix-map= for generated assembly debug info (DWARF v5)

For generated assembly debug info, MCDwarfLineTableHeader::CompilationDir is an
unmapped path set in MCContext::setGenDwarfRootFile. Remap it.

A relative destination path of -fdebug-prefix-map= exposes a llvm-dwarfdump bug
which joins relative DW_AT_comp_dir and directories[0].

Fix https://github.com/llvm/llvm-project/issues/56609

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D131749