c++: Local instantiations of imported specializations [PR 99377]

c++: Local instantiations of imported specializations [PR 99377]

This turned out to be the function version of the previous fix.  We
can import an implicit specialization declaration that we need to
instantiate.  We must mark the instantiation so we remember to stream
it.

	PR c++/99377
	gcc/cp/
	* pt.c (instantiate_decl): Call set_instantiating_module.
	gcc/testsuite/
	* g++.dg/modules/pr99377_a.H: New.
	* g++.dg/modules/pr99377_b.C: New.
	* g++.dg/modules/pr99377_c.C: New.