compiler: add src information to AST dumps (part 1 of 2).

compiler: add src information to AST dumps (part 1 of 2).

    When emitting AST dumps, tag the end of selected statements with with
    source file and line number information where available. Example:

      tmp.76832448 = 0 // p.go:6

    Requires a corresponding change in gcc/go as well as this change to
    gofrontend.

    Reviewed-on: https://go-review.googlesource.com/29856

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

From-SVN: r240558