c++: more mce_false folding from cp_fully_fold_init [PR108243]

We should also fold the overall initializer passed to cp_fully_fold_init
with mce_false, which allows folding of the copy-initialization of
'a1' in the below testcase (the initializer here is an AGGR_INIT_EXPR).

Unfortunately this doesn't help with direct- or default-initialization
because we don't call cp_fully_fold_init in that case, and even if we
did the initializer in that case is expressed as a bare CALL_EXPR
instead of an AGGR_INIT_EXPR, which cp_fully_fold_init can't really
fold.

	PR c++/108243
	PR c++/97553

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fully_fold): Add an internal overload that
	additionally takes and propagate an mce_value parameter, and
	define the existing public overload in terms of it.
	(cp_fully_fold_init): Pass mce_false to cp_fully_fold.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/is_constant_evaluated3.C: New test.