c++: header unit purview [PR 99283]

c++: header unit purview [PR 99283]

This case occurs due to some equivocation about module_purview.
Header-unit building is treated as a module-purview, but we should not
treat entities imported from that as module purview.  (header units
were not a thing when I started).  The testcase didn't understand we
had a local textual definition, but it was (incorrectly) marked as
module-purview, because we'd read in a declaration from a header unit
too.

	gcc/cp/
	* cp-tree.h (lang_decl_base): Correct module flag comment.
	* module.cc (trees_in::assert_definition): Break out
	not_tmpl var.
	(trees_out::lang_decl_bools): Do not write purview for header units.
	gcc/testsuite/
	* g++.dg/modules/pr99283-6_d.H: New.
	* g++.dg/modules/pr99283-7-swap.h: New.
	* g++.dg/modules/pr99283-7-traits.h: New.
	* g++.dg/modules/pr99283-7_a.H: New.
	* g++.dg/modules/pr99283-7_b.H: New.
	* g++.dg/modules/pr99283-7_c.C: New.
	* g++.dg/modules/pr99283-7_d.H: New.