P0595R1 - is_constant_evaluated
cp/
	* cp-tree.h (enum cp_built_in_function): New.
	(maybe_constant_init): Add pretend_const_required argument.
	* typeck2.c (store_init_value): Pass true as new argument to
	maybe_constant_init.
	* constexpr.c (constexpr_fn_retval): Check also DECL_BUILT_IN_CLASS
	for BUILT_IN_UNREACHABLE.
	(struct constexpr_ctx): Add pretend_const_required field.
	(cxx_eval_builtin_function_call): Use DECL_IS_BUILTIN_CONSTANT_P
	macro.  Handle CP_BUILT_IN_IS_CONSTANT_EVALUATED.  Check also
	DECL_BUILT_IN_CLASS for BUILT_IN_UNREACHABLE.
	(cxx_eval_outermost_constant_expr): Add pretend_const_required
	argument, initialize pretend_const_required field in ctx.  If the
	result is TREE_CONSTANT and non_constant_p, retry with
	pretend_const_required false if it was true.
	(is_sub_constant_expr): Initialize pretend_const_required_field in
	ctx.
	(cxx_constant_value): Pass true as pretend_const_required to
	cxx_eval_outermost_constant_expr.
	(maybe_constant_value): Pass false as pretend_const_required to
	cxx_eval_outermost_constant_expr.
	(fold_non_dependent_expr): Likewise.
	(maybe_constant_init_1): Add pretend_const_required argument, pass it
	down to cxx_eval_outermost_constant_expr.  Pass !allow_non_constant
	instead of false as strict to cxx_eval_outermost_constant_expr.
	(maybe_constant_init): Add pretend_const_required argument, pass it
	down to maybe_constant_init_1.
	(cxx_constant_init): Pass true as pretend_const_required to
	maybe_constant_init_1.
	* cp-gimplify.c (cp_gimplify_expr): Handle CALL_EXPRs to
	CP_BUILT_IN_IS_CONSTANT_EVALUATED.
	(cp_fold): Don't fold CP_BUILT_IN_IS_CONSTANT_EVALUATED calls.
	* decl.c: Include langhooks.h.
	(cxx_init_decl_processing): Register __builtin_is_constant_evaluated
	built-in.
	* tree.c (builtin_valid_in_constant_expr_p): Return true for
	CP_BUILT_IN_IS_CONSTANT_EVALUATED.
	* pt.c (declare_integer_pack): Initialize DECL_FUNCTION_CODE.
testsuite/
	* g++.dg/cpp2a/is-constant-evaluated1.C: New test.

From-SVN: r263392