c++: header unit template alias merging [PR 98770]

Typedefs are streamed by streaming the underlying type, and then
recreating the typedef.  But this breaks checking a duplicate is the
same as the original when it is a template alias -- we end up checking
a template alias (eg __void_t) against the underlying type (void).
And those are not the same template alias.  This stops pretendig that
the underlying type is the typedef for that checking and tells
is_matching_decl 'you have a typedef', so it knows what to do.  (We do
not want to recreate the typedef of the duplicate, because that whole
set of nodes is going to go away.)

	PR c++/98770
	gcc/cp/
	* module.cc (trees_out::decl_value): Swap is_typedef & TYPE_NAME
	check order.
	(trees_in::decl_value): Do typedef frobbing only when installing
	a new typedef, adjust is_matching_decl call.  Swap is_typedef
	& TYPE_NAME check.
	(trees_in::is_matching_decl): Add is_typedef parm. Adjust variable
	names and deal with typedef checking.
	gcc/testsuite/
	* g++.dg/modules/pr98770_a.C: New.
	* g++.dg/modules/pr98770_b.C: New.

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