c++: cross-header-unit template definitions [PR 99153]

c++: cross-header-unit template definitions [PR 99153]

A member function can be defined in a different header-file than the
one defining the class.  In such situations we must unmark the decl as
imported.  When the entity is a template we failed to unmark the
template_decl.

Perhaps the duplication of these flags on the template_decl from the
underlying decl is an error.  I set on the fence about it for a long
time during development, but I don't think now is the time to change
that (barring catastrophic bugs).

	PR c++/99153
	gcc/cp/
	* decl.c (duplicate_decls): Move DECL_MODULE_IMPORT_P propagation
	to common-path.
	* module.cc (set_defining_module): Add assert.
	gcc/testsuite/
	* g++.dg/modules/pr99153_a.H: New.
	* g++.dg/modules/pr99153_b.H: New.