c++: always check consteval address

c++: always check consteval address

The restriction on the "permitted result of a constant expression" to not
refer to an immediate function applies regardless of context.  The previous
code tried to only check in cases where we wouldn't get the check in
cp_fold_r, but with the next patch I would need to add another case and it
shouldn't be a problem to always check.

We also shouldn't talk about immediate evaluation when we aren't dealing
with one.

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_outermost_constant_expr): Always check
	for address of immediate fn.
	(maybe_constant_init_1): Evaluate PTRMEM_CST.

gcc/testsuite/ChangeLog:

	* g++.dg/DRs/dr2478.C: Handle -fimplicit-constexpr.
	* g++.dg/cpp23/consteval-if12.C: Adjust diagnostics.
	* g++.dg/cpp2a/consteval20.C: Likewise.
	* g++.dg/cpp2a/consteval24.C: Likewise.
	* g++.dg/cpp2a/srcloc20.C: Likewise.