c++/modules: Fix imported CNTTPs being considered non-constant [PR119938]

When importing a CNTTP object, since r15-3031-g0b7904e274fbd6 we
shortcut the processing of the generated NTTP so that we don't attempt
to recursively load pendings.  However, due to an oversight we do not
properly set TREE_CONSTANT or DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P
on the decl, which confuses later processing.  This patch ensures that
this happens correctly.

	PR c++/119938

gcc/cp/ChangeLog:

	* pt.cc (get_template_parm_object): When !check_init, add assert
	that expr really is constant and mark decl as such.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/tpl-nttp-2_a.H: New test.
	* g++.dg/modules/tpl-nttp-2_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit d613678c94f06809656e56b37f314501b37a5ddd)