debug/101473 - apply debug prefix maps before checksumming DIEs

debug/101473 - apply debug prefix maps before checksumming DIEs

The following makes sure to apply the debug prefix maps to filenames
before checksumming DIEs to create the global symbol for the CU DIE
used by LTO to link the late debug to the early debug.  This avoids
binary differences (in said symbol) when compiling with toolchains
installed under a different path and that compensated with appropriate
-fdebug-prefix-map options.

The easiest and most scalable way is to record both the unmapped
and the remapped filename in the dwarf_file_data so the remapping
process takes place at a single point and only once (otherwise it
creates GC garbage at each point doing that).

2021-07-20  Richard Biener  <rguenther@suse.de>

	PR debug/101473
	* dwarf2out.h (dwarf_file_data): Add key member.
	* dwarf2out.c (dwarf_file_hasher::equal): Compare key.
	(dwarf_file_hasher::hash): Hash key.
	(lookup_filename): Remap the filename and store it in the
	filename member of dwarf_file_data when creating a new
	dwarf_file_data.
	(file_name_acquire): Do not remap the filename again.
	(maybe_emit_file): Likewise.