PR c++/91476 - anon-namespace reference temp clash between TUs.

PR c++/91476 - anon-namespace reference temp clash between TUs.

The problem in the PR was that make_temporary_var_for_ref_to_temp ran before
determine_visibility, so when we copied the linkage of the reference
variable it had not yet been restricted by its anonymous namespace context,
so the temporary wrongly ended up with TREE_PUBLIC set.  The natural
solution is to run determine_visibility earlier.  But that needs to happen
after maybe_commonize_var increases the linkage of some local variables, and
on targets without weak symbol support, that function does different things
based on the results of check_initializer, which is what calls
make_temporary_var_for_ref_to_temp.  To break this circular dependency I'm
calling maybe_commonize_var early, and then again later if the target
doesn't support weak symbols.

It also occurred to me that make_temporary_var_for_ref_to_temp wasn't
handling DECL_VISIBILITY at all, and verified that we were doing the wrong
thing.  So I've combined the linkage-copying code from there and two other
places.

	* decl2.c (copy_linkage): Factor out of get_guard.
	* call.c (make_temporary_var_for_ref_to_temp): Use it.
	* decl.c (cp_finish_decomp): Use it.
	(cp_finish_decl): determine_visibility sooner.