fix -fdebug-prefix-map without gas .file support

fix -fdebug-prefix-map without gas .file support

This applies file mapping when emitting the directory table
directly instead of using the assemblers .file directive where
we already correctly apply the map.  Notably the non-assembler
path is used for the early debug emission for LTO.

2020-02-27  Mark Williams  <mwilliams@fb.com>

	* dwarf2out.c (file_name_acquire): Call remap_debug_filename.
	* lto-opts.c (lto_write_options): Drop -fdebug-prefix-map,
	-ffile-prefix-map and -fmacro-prefix-map.
	* lto-streamer-out.c: Include file-prefix-map.h.
	(lto_output_location): Remap the file part of locations.