c++/modules: inline loaded at eof

c++/modules: inline loaded at eof

std/format/string.cc and a few other libstdc++ tests were failing with
module std with undefined references to __failed_to_parse_format_spec.  This
turned out to be because since r15-8012 we don't end up calling
note_vague_linkage_fn for functions loaded after at_eof is set.

But once import_export_decl decides on COMDAT linkage, we should be able to
just clear DECL_EXTERNAL and let cgraph take it from there.

I initially made this change in import_export_decl, but decided that for GCC
15 it would be safer to limit the change to modules.  For GCC 16 I'd like to
do away with DECL_NOT_REALLY_EXTERN entirely, it's been obsolete since
cgraphunit in 2003.

gcc/cp/ChangeLog:

	* module.cc (module_state::read_cluster)
	(post_load_processing): Clear DECL_EXTERNAL if DECL_COMDAT.