compiler: Define and use backend-independent Location class.

compiler: Define and use backend-independent Location class.

From Sanjoy Das.

	* go-location.h: New file.
	* go-linemap.cc: New file.
	* go-gcc.cc: Change all uses of source_location to Location.
	* Make-lang.in (GO_OBJS): Add go/go-linemap.o.
	(GO_LINEMAP_H): New variable.
	(GO_LEX_H): Use $(GO_LINEMAP_H).
	(GO_GOGO_H, GO_TYPES_H, GO_IMPORT_H): Likewise.
	(go/go-linemap.o): New target.

Co-Authored-By: Ian Lance Taylor <iant@google.com>
From-SVN: r181813