c++: ICE do to GC leakage [PR 99423]

c++: ICE do to GC leakage [PR 99423]

My reworking of pending-entity loading introduced a GC problem.  The
post-load processing needs to inhibit GCs (that would otherwise occur
in clone_decl).  That wasn't happening on one code path, leading to
dangling pointers in the active call frames.

	PR c++/99423
	gcc/cp/
	* module.cc (post_load_processing): Assert not gcable.
	(laxy_load_pendings): Extend no-gc region around
	post_load_processing.
	gcc/testsuite/
	* g++.dg/modules/pr99423_a.H: New.
	* g++.dg/modules/pr99423_b.H: New.