c++/modules: Disable streaming definitions of non-vague-linkage GMF decls [PR115020]

The error in the linked PR is caused because 'DECL_THIS_STATIC' is true
for the static member function, causing the streaming code to assume
that this is an internal linkage GM entity that needs to be explicitly
streamed, which then on read-in gets marked as a vague linkage function
(despite being non-inline) causing import_export_decl to complain.

However, I don't see any reason why we should care about this:
definitions in the GMF should just be emitted as per usual regardless of
whether they're internal-linkage or not.  Actually the only thing we
care about here are header modules, since they have no TU to write
definitions into.  As such this patch removes these conditions from
'has_definition' and updates some comments to clarify.

	PR c++/115020

gcc/cp/ChangeLog:

	* module.cc (has_definition): Only force writing definitions for
	header_module_p.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr115020_a.C: New test.
	* g++.dg/modules/pr115020_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>