c++: extern thread_local declarations in constexpr [PR104994]

C++14 to C++20 apparently should allow extern thread_local declarations in
constexpr functions, however useless they are there (because accessing
such vars is not valid in a constant expression, perhaps sizeof/decltype).
P2242 changed that for C++23 to passing through declaration but
https://cplusplus.github.io/CWG/issues/2552.html
has been filed for it yesterday.

The following patch implements the proposed wording of CWG 2552 in addition
to fixing the C++14 - C++20 handling bug.
If you'd like instead to keep the current pedantic C++23 wording for now,
that would mean taking out the first hunk (cxx_eval_constant_expression) and
g++.dg/cpp23/constexpr-nonlit2.C hunk.

2022-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104994
	* constexpr.cc (cxx_eval_constant_expression): Don't diagnose passing
	through extern thread_local declarations.  Change wording from
	declaration to definition.
	(potential_constant_expression_1): Don't diagnose extern thread_local
	declarations.  Change wording from declared to defined.
	* decl.cc (start_decl): Likewise.

	* g++.dg/diagnostic/constexpr1.C: Change expected diagnostic wording
	from declared to defined.
	* g++.dg/cpp23/constexpr-nonlit1.C: Likewise.
	(garply): Change dg-error into dg-bogus.
	* g++.dg/cpp23/constexpr-nonlit2.C: Change expected diagnostic wording
	from declaration to definition.
	* g++.dg/cpp23/constexpr-nonlit6.C: Change expected diagnostic wording
	from declared to defined.
	* g++.dg/cpp23/constexpr-nonlit7.C: New test.
	* g++.dg/cpp2a/constexpr-try5.C: Change expected diagnostic wording
	from declared to defined.
	* g++.dg/cpp2a/consteval3.C: Likewise.