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.