c++: Check module attachment instead of just purview when necessary [PR112631]

Block-scope declarations of functions or extern values are not allowed
when attached to a named module. Similarly, class member functions are
not inline if attached to a named module. However, in both these cases
we currently only check if the declaration is within the module purview;
it is possible for such a declaration to occur within the module purview
but not be attached to a named module (e.g. in an 'extern "C++"' block).
This patch makes the required adjustments.

	PR c++/112631

gcc/cp/ChangeLog:

	* cp-tree.h (named_module_attach_p): New function.
	* decl.cc (start_decl): Check for attachment not purview.
	(grokmethod): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/block-decl-1_a.C: New test.
	* g++.dg/modules/block-decl-1_b.C: New test.
	* g++.dg/modules/block-decl-2.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>