c++: Fix bogus warnings about ignored annotations [PR114691]

c++: Fix bogus warnings about ignored annotations [PR114691]

The middle-end warns about the ANNOTATE_EXPR added for while/for loops
if they declare a var inside of the loop condition.
This is because the assumption is that ANNOTATE_EXPR argument is used
immediately in a COND_EXPR (later GIMPLE_COND), but simplify_loop_decl_cond
wraps the ANNOTATE_EXPR inside of a TRUTH_NOT_EXPR, so it no longer
holds.

The following patch fixes that by adding the TRUTH_NOT_EXPR inside of the
ANNOTATE_EXPR argument if any.

2024-04-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/114691
	* semantics.cc (simplify_loop_decl_cond): Use cp_build_unary_op with
	TRUTH_NOT_EXPR on ANNOTATE_EXPR argument (if any) rather than
	ANNOTATE_EXPR itself.

	* g++.dg/ext/pr114691.C: New test.