c++/modules: Prevent maybe_clone_decl being called multiple times [PR115007]

c++/modules: Prevent maybe_clone_decl being called multiple times [PR115007]

The ICE in the linked PR is caused because maybe_clone_decl is not
prepared to be called on a declaration that has already had clones
created; what happens otherwise is that start_preparsed_function early
exits and never sets up cfun, causing a segfault later on.

To fix this we ensure that post_load_processing only calls
maybe_clone_decl if TREE_ASM_WRITTEN has not been marked on the
declaration yet, and (if maybe_clone_decls succeeds) marks this flag on
the decl so that it doesn't get called again later when finalising
deferred vague linkage declarations in c_parse_final_cleanups.

As a bonus this now allows us to only keep the DECL_SAVED_TREE around in
expand_or_defer_fn_1 for modules which have CMIs, which will have
benefits for LTO performance in non-interface TUs.

For clarity we also update the streaming code to do post_load_decls for
maybe in-charge cdtors rather than any DECL_ABSTRACT_P declaration, as
this is more accurate to the decls affected by maybe_clone_body.

	PR c++/115007

gcc/cp/ChangeLog:

	* module.cc (module_state::read_cluster): Replace
	DECL_ABSTRACT_P with DECL_MAYBE_IN_CHARGE_CDTOR_P.
	(post_load_processing): Check and mark TREE_ASM_WRITTEN.
	* semantics.cc (expand_or_defer_fn_1): Use the more specific
	module_maybe_has_cmi_p instead of modules_p.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/virt-6_a.C: New test.
	* g++.dg/modules/virt-6_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>