c++: mangling cleanups

c++: mangling cleanups

I noticed the mangler's handling of templates could be simplified.
We know template_info is non-null, which is sufficiently boolean --
no need for an explicit bool return.  also some of the internals of
template_args_equal had crept into find_substitution.  Let's not do
that.

	gcc/cp/
	* mangle.c (decl_is_template_id): Rename to ...
	(maybe_template_info): ... here.  Return the template info,
	rather than use a pointer.  Adjust all callers.
	(find_substitution): Use template_args_equal, rather than
	local check.