Further increase the limit on the size of accepted Ada files

It turns out that the limit on the size of accepted Ada files had been
already lowered earlier, namely when location ranges had been introduced.
Now we do not make use of location ranges in Ada so we can recoup the loss.

gcc/ada/
	* gcc-interface/misc.c (gnat_init): Set default range bits to 0.
	* gcc-interface/trans.c (extract_encoding): Delete.
	(decode_name): Likewise.
	(File_Name_to_gnu): New function.
	(gigi): Call it to translate file names.  Replace assertion on
	1-1 mapping between files and line maps with conditional error.