c++: template instantiation during fold_for_warn [PR94038]

Unfortunately, the previous fix to PR94038 is fragile.  When the
argument to fold_for_warn is a bare CALL_EXPR, then all is well: the
result of maybe_constant_value from fold_for_warn (with
uid_sensitive=true) is reused via the cv_cache in the subsequent call to
maybe_constant_value from cp_fold (with uid_sensitive=false), so we
avoid instantiating bar<int>.

But when the argument to fold_for_warn is more complex, e.g. an
INDIRECT_REF of a CALL_EXPR, as in the testcase below (due to bar<int>()
returning const int& which we need to decay to int) then from
fold_for_warn we call maybe_constant_value on the INDIRECT_REF, and from
cp_fold we call it on the CALL_EXPR, so there is no reuse via the
cv_cache and we therefore end up instantiating bar<int>.

So for a more robust solution to this general issue of warning flags
affecting code generation, it seems that we need a way to globally avoid
template instantiation during constexpr evaluation whenever we're
performing warning-dependent folding.

To that end, this patch replaces the flag constexpr_ctx::uid_sensitive
with a global flag uid_sensitive_constexpr_evaluation_p, and enables it
during fold_for_warn using an RAII helper.

The patch also adds a counter that keeps track of the number of times
uid_sensitive_constexpr_evaluation_p is called and returned true, and we
use this to determine whether the result of constexpr evaluation
was restricted by the flag.  This lets us safely update the cv_cache and
fold_cache from fold_for_warn in the most common case where the flag
did not restrict constexpr evaluation.

gcc/cp/ChangeLog:

	PR c++/94038
	* constexpr.c (constexpr_ctx::uid_sensitive): Remove field.
	(uid_sensitive_constexpr_evaluation_value): Define.
	(uid_sensitive_constexpr_evaluation_true_counter): Define.
	(uid_sensitive_constexpr_evaluation_p): Define.
	(uid_sensitive_constexpr_evaluation_sentinel): Define its
	constructor.
	(uid_sensitive_constexpr_evaluation_checker): Define its
	constructor and its evaluation_restricted_p method.
	(get_fundef_copy): Remove 'ctx' parameter.  Use u_s_c_e_p
	instead of constexpr_ctx::uid_sensitive.
	(cxx_eval_call_expression): Use u_s_c_e_p instead, and test it
	last.  Adjust call to get_fundef_copy.
	(instantiate_cx_fn_r): Test u_s_c_e_p so that we increment the
	counter if necessary.
	(cxx_eval_outermost_constant_expr): Remove 'uid_sensitive'
	parameter.  Adjust function body accordingly.
	(maybe_constant_value): Remove 'uid_sensitive' parameter and
	adjust function body accordingly.  Set up a
	uid_sensitive_constexpr_evaluation_checker, and use it to
	conditionally update the cv_cache.
	* cp-gimplify.c (cp_fold): Set up a
	uid_sensitive_constexpr_evaluation_checker, and use it to
	conditionally update the fold_cache.
	* cp-tree.h (maybe_constant_value): Update declaration.
	(struct uid_sensitive_constexpr_evaluation_sentinel): Define.
	(struct sensitive_constexpr_evaluation_checker): Define.
	* expr.c (fold_for_warn): Set up a
	uid_sensitive_constexpr_evaluation_sentinel before calling
	the folding subroutines.  Drop all but the first argument to
	maybe_constant_value.

gcc/testsuite/ChangeLog:

	PR c++/94038
	* g++.dg/warn/pr94038-2.C: New test.