c++/modules: Avoid rechecking initializers when streaming NTTPs [PR116382]

When reading an NTTP we call get_template_parm_object which delegates
setting of DECL_INITIAL to the general cp_finish_decl procedure, which
calls check_initializer to validate and record it.

Apart from being unnecessary (it must have already been validated by the
writing module), this also causes errors in cases like the linked PR, as
validating may end up needing to call lazy_load_pendings to determine
any specialisations that may exist which violates assumptions of the
modules streaming code.

This patch works around the issue by adding a flag to
get_template_parm_object to disable these checks when not needed.

	PR c++/116382

gcc/cp/ChangeLog:

	* cp-tree.h (get_template_parm_object): Add check_init param.
	* module.cc (trees_in::tree_node): Pass check_init=false when
	building NTTPs.
	* pt.cc (get_template_parm_object): Prevent cp_finish_decl from
	validating the initializer when check_init=false.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/tpl-nttp-1_a.C: New test.
	* g++.dg/modules/tpl-nttp-1_b.C: New test.

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