Add an interface to the go Linemap class to allow clients to ask for the
    line number for a Location (for debugging dumps), so as to move some
    gcc-specific location code into the back end.
    
    Reviewed-on: https://go-review.googlesource.com/30699

	* go-linemap.cc (Gcc_linemap::location_line): New method.

From-SVN: r240942