c++: Use constexpr to avoid wrong -Wsign-compare (PR90691).

c++: Use constexpr to avoid wrong -Wsign-compare (PR90691).

We would like to do constexpr evaluation to avoid false positives on
warnings, but constexpr evaluation can involve function body copying that
changes DECL_UID, which breaks -fcompare-debug.  So let's remember
that we need to avoid that.

	PR c++/90691
	* expr.c (fold_for_warn): Call maybe_constant_value.
	* constexpr.c (struct constexpr_ctx): Add uid_sensitive field.
	(maybe_constant_value): Add uid_sensitive parm.
	(get_fundef_copy): Don't copy if it's true.
	(cxx_eval_call_expression): Don't instantiate if it's true.
	(cxx_eval_outermost_constant_expr): Likewise.