Fix mangling ICE [PR94027]

Fix mangling ICE [PR94027]

	PR c++/94027
	* mangle.c (find_substitution): Don't call same_type_p on template
	args that cannot match.

Now same_type_p rejects argument packs, we need to be more careful
calling it with template argument vector contents.

The mangler needs to do some comparisons to find the special
substitutions.  While that code looks a little ugly, this seems the
smallest fix.