c++: speculative constexpr and is_constant_evaluated [PR108243]

c++: speculative constexpr and is_constant_evaluated [PR108243]

This PR illustrates that __builtin_is_constant_evaluated currently acts
as an optimization barrier for our speculative constexpr evaluation,
since we don't want to prematurely fold the builtin to false before the
expression in question undergoes manifestly constant evaluation if
appropriate (in which case the builtin must instead be folded to true).

This patch fixes this by permitting __builtin_is_constant_evaluated to
get folded as false at appropiate points, namely during cp_fold_function
and cp_fully_fold_init where we know we're done with manifestly constant
evaluation.  The function cp_fold gets a flags parameter that controls
whether we pass mce_false or mce_unknown to maybe_constant_value when
folding a CALL_EXPR.

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

gcc/cp/ChangeLog:

	* cp-gimplify.cc (enum fold_flags): Define.
	(fold_flags_t): Declare.
	(cp_fold_data::genericize): Replace this data member with ...
	(cp_fold_data::fold_flags): ... this.
	(cp_fold_r): Adjust use of cp_fold_data and calls to cp_fold.
	(cp_fold_function): Likewise.
	(cp_fold_maybe_rvalue): Add an internal overload that
	additionally takes and propagates a fold_flags_t parameter, and
	define the existing public overload in terms of it.
	(cp_fold_rvalue): Likewise.
	(cp_fully_fold_init): Adjust use of cp_fold_data.
	(fold_cache): Replace with ...
	(fold_caches): ... this 2-element array of caches.
	(get_fold_cache): Define.
	(clear_fold_cache): Adjust.
	(cp_fold): Add fold_flags_t parameter.  Use get_fold_cache.
	Pass flags to calls to cp_fold, cp_fold_rvalue and
	cp_fold_maybe_rvalue.
	<case CALL_EXPR>: If ff_mce_false is set, fold
	__builtin_is_constant_evaluated to false and pass mce_false to
	maybe_constant_value.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/is_constant_evaluated1.C: New test.
	* g++.dg/opt/is_constant_evaluated2.C: New test.