c++: cleanup function name [PR 98531]

c++: cleanup function name [PR 98531]

The next piece of 98531 is that in some cases we need to create a
cleanup function to do the work (when the object is an array, or we're
using regular atexit).  We were not pushing that function's decl
anywhere (not giving it a context) so streaming it failed.

This is a partial fix.  You'll notice we're naming these from a per-TU
counter.  I've captured that in PR98893.

	gcc/cp/
	* decl.c (start_cleanup_fn): Push function into
	namespace.
	gcc/testsuite/
	* g++.dg/modules/pr98531-2.h: New.
	* g++.dg/modules/pr98531-2_a.H: New.
	* g++.dg/modules/pr98531-2_b.C: New.
	* g++.dg/modules/pr98531-3.h: New.
	* g++.dg/modules/pr98531-3_a.H: New.
	* g++.dg/modules/pr98531-3_b.C: New.