compiler: better abstraction layer for diagnostics.

compiler: better abstraction layer for diagnostics.

    Introduce an abstraction layer for reporting diagnostics, so as to avoid
    directly using the native GCC interfaces such as "error_at",
    "warning_at", "open_quote", "close_quote", etc.  The new interfaces have
    the same look and feel as the GCC equivalents, but make calls into
    back-end functions to allow the back end to select the proper final
    reporting routine.

    Reviewed-on: https://go-review.googlesource.com/29191

	* go-gcc-diagnostics.cc: New file.
	* go-location.h (Location): Remove operator source_location.  Add
	operator==.
	* go-system.h: #include <sstream>.
	* Make-lang.in (GO_OBJS): Add go/go-diagnostics.o and
	go/go-gcc-diagnostics.o.
	(CFLAGS-go/go-gcc-diagnostics.o): New variable.

From-SVN: r240453