cpplib: Macro use location and comparison

Our macro use hook passes a location, but doesn't recieve it from the
using location.  This patch adds the extra location_t parameter and
passes it though.

A second cleanup is breaking out the macro comparison code from the
redefinition warning.  That;ll turn out useful for modules.

Finally, there's a filename comparison needed for the location
optimization of rewinding from line 2 (occurs during the emission of
builtin macros).

	libcpp/
	* internal.h (_cpp_notify_macro_use): Add location parm.
	(_cpp_maybe_notify_macro_use): Likewise.
	* directives.c (_cpp_do_file_change): Check we've not changed file
	when optimizing a rewind.
	(do_ifdef): Pass location to _cpp_maybe_notify_macro_use.
	(do_ifndef): Likewise.  Delete obsolete comment about powerpc.
	* expr.c (parse_defined): Pass location to
	_cpp_maybe_notify_macro_use.
	* macro.c (enter_macro_context): Likewise.
	(warn_of_redefinition): Break out helper function.  Call it.
	(compare_macros): New function broken out of warn_of_redefinition.
	(_cpp_new_macro): Zero all fields.
	(_cpp_notify_macro_use): Add location parameter.

libcpp/expr.c

@@ -1070,7 +1070,7 @@ parse_defined (cpp_reader *pfile)
 		        "this use of \"defined\" may not be portable");
 
       _cpp_mark_macro_used (node);
-      _cpp_maybe_notify_macro_use (pfile, node);
+      _cpp_maybe_notify_macro_use (pfile, node, token->src_loc);
 
       /* A possible controlling macro of the form #if !defined ().
 	 _cpp_parse_expr checks there was no other junk on the line.  */