c++: Implement C++23 P2647R1 - Permitting static constexpr variables in constexpr...

c++: Implement C++23 P2647R1 - Permitting static constexpr variables in constexpr functions

The following patch implements this paper.
Per further discussions it is implemented for C++23 only, so isn't
treated as a DR, e.g. because the part of the standard the paper is
changing didn't even exist in C++20.
And we gave up on trying to implement it as a pedwarn rather than
error for C++20 and older, because of implicit constexpr lambdas or
-fimplicit-constexpr reasons.
For C++20 and older, the only change is that passing through
definitions of static or thread_local vars usable in constant expressions
is now accepted in statement expressions if they aren't inside of constexpr
or consteval functions.

2022-11-19  Jakub Jelinek  <jakub@redhat.com>

gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Bump __cpp_constexpr
	value from 202207L to 202211L.
gcc/cp/
	* constexpr.cc (cxx_eval_constant_expression): Implement C++23
	P2647R1 - Permitting static constexpr variables in constexpr functions.
	Allow DECL_EXPRs of decl_constant_var_p static or thread_local vars.
	(potential_constant_expression_1): Similarly, except use
	decl_maybe_constant_var_p instead of decl_constant_var_p if
	processing_template_decl.
gcc/testsuite/
	* g++.dg/cpp23/constexpr-nonlit17.C: New test.
	* g++.dg/cpp23/constexpr-nonlit18.C: New test.
	* g++.dg/cpp23/feat-cxx2b.C: Adjust expected __cpp_constexpr
	value.
	* g++.dg/ext/stmtexpr19.C: Don't expect an error.
	* g++.dg/ext/stmtexpr25.C: New test.