c++: Macros need to be GTY-reachable [PR 99023]

c++: Macros need to be GTY-reachable [PR 99023]

I'd missed that macros were allocated from GC storage, and that they can
become unattached from an identifier, and therefore not GC-reachable.
And then bad things happen.   Fixed by making the module machinery's
reference vector a GC root.

	PR c++/99023
	gcc/cp/
	* module.cc (struct macro_export): Add GTY markers.
	(macro_exports): Likewise, us a va_gc Vector.
	gcc/testsuite/
	* g++.dg/modules/pr99023_a.H: New.
	* g++.dg/modules/pr99023_b.H: New.