c++: Implement __builtin_constexpr_diag (aka P2758 implementation)

c++: Implement __builtin_constexpr_diag (aka P2758 implementation)

I know P2758R5 didn't make it into C++26, but on IRC Ville said it would be
useful anyway, so here is a quick attempt at implementing it.

Not adding anything on the libstdc++ side, because I don't know where
experimental stuff like that should go, whether it would be in the
implementation namespace etc.

Tags are currently parsed and printed in the diagnostics, but -Wconstexpr-msg=
option and being able to #pragma GCC diagnostic ignore -Wconstexpr-msg=blah
will need to wait for GCC 17.

In any case, the compiler side is just one new builtin,
__builtin_constexpr_diag, whose first argument is 0 for print,
or 1 for warning or 2 for error (or that ored with 16 if location shouldn't
be caller's location but caller's caller location - useful when adding some
wrappers around the builtin), second argument is string_view of the tag
(or "" for no tag) and third argument is string_view or u8string_view
with the message.  The builtin also handles string literals.

2026-02-17  Jakub Jelinek  <jakub@redhat.com>

gcc/cp/
	* cp-tree.h (enum cp_built_in_function): Add
	CP_BUILT_IN_CONSTEXPR_DIAG.
	(cexpr_str::cexpr_str): Add new default ctor.
	(cexpr_str::type_check): Add optional allow_char8_t arg.
	(cexpr_str::extract): Add optional ctx, non_constant_p, overflow_p
	and jump_target arguments.
	* cp-gimplify.cc (cp_gimplify_expr): Throw away
	__builtin_constexpr_diag calls after gimplification of
	their arguments.
	* decl.cc (cxx_init_decl_processing): Create __builtin_constexpr_diag
	FE builtin decl.
	* constexpr.cc (call_stack, call_stack_tick, last_cx_error_tick):
	Moved earlier.
	(cxx_eval_constexpr_diag): New function.
	(cxx_eval_builtin_function_call): Handle __builtin_constexpr_diag
	calls.
	* tree.cc (builtin_valid_in_constant_expr_p): Return true for
	CP_BUILT_IN_CONSTEXPR_DIAG.
	* semantics.cc (cexpr_str::type_check): Add allow_char8_t argument,
	if true, allow data to return const char8_t *.
	(cexpr_str::extract): Add ctx, non_constant_p, overflow_p and
	jump_target arguments, if they are non-NULL, evalute expressions
	in that context rather than using cxx_constant_value and translate
	back to SOURCE_CHARSET even if message_data or message_sz are NULL.
gcc/testsuite/
	* g++.dg/ext/constexpr-diag1.C: New test.
	* g++.dg/ext/constexpr-diag2.C: New test.
	* g++.dg/ext/constexpr-diag3.C: New test.
	* g++.dg/ext/constexpr-diag4.C: New test.
	* g++.dg/ext/constexpr-diag5.C: New test.
	* g++.dg/ext/constexpr-diag6.C: New test.