c++: missing SFINAE for non-constant consteval calls [PR104620]

Here we weren't respecting SFINAE when evaluating a call to a consteval
function, which caused us to reject the new testcase below.  This patch
fixes this by making build_over_call use the SFINAE-friendly version of
cxx_constant_value.

This change causes us to no longer diagnose ahead of time a couple of
non-constant non-dependent consteval calls in consteval-if2.C with
-fchecking=2.  These errors were apparently coming from the call to
fold_non_dependent_expr in build_non_dependent_expr (for the RHS of the +=)
despite complain=tf_none being passed.  Now that build_over_call respects
the value of complain during constant evaluation of a consteval call,
the errors are gone.

That the errors are also gone without -fchecking=2 is a regression caused
by r12-7264-gc19f317a and is the subject of PR104620.  As described
in comment #5, I think it's basically an accident that we were diagnosing
these two calls correctly before r12-7264, so perhaps we can live without
these errors for GCC 12.  Thus this patch just XFAILs the two tests.

	PR c++/104620

gcc/cp/ChangeLog:

	* call.cc (build_over_call): Use cxx_constant_value_sfinae
	instead of cxx_constant_value to evaluate a consteval call.
	* constexpr.cc (cxx_constant_value_sfinae): Add decl parameter
	and pass it to cxx_eval_outermost_constant_expr.
	* cp-tree.h (cxx_constant_value_sfinae): Add decl parameter.
	* pt.cc (fold_targs_r): Pass NULL_TREE as decl parameter to
	cxx_constant_value_sfinae.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/consteval-if2.C: XFAIL two dg-error tests where
	the argument to the non-constant non-dependent consteval call is
	wrapped by NON_DEPENDENT_EXPR.
	* g++.dg/cpp2a/consteval30.C: New test.