c++: Don't emit deprecated/unavailable attribute diagnostics when creating cdtor...

c++: Don't emit deprecated/unavailable attribute diagnostics when creating cdtor thunks [PR116678]

Another spot where we mark_used a function (in this case ctor or dtor)
even when it is just artificially used inside of thunks (emitted on mingw
with -Os for the testcase).

2024-09-13  Jakub Jelinek  <jakub@redhat.com>

	PR c++/116678
	* optimize.cc: Include decl.h.
	(maybe_thunk_body): Temporarily change deprecated_state to
	UNAVAILABLE_DEPRECATED_SUPPRESS.

	* g++.dg/warn/deprecated-20.C: New test.

(cherry picked from commit b7b67732e20217196f2a13a10fc3df4605b2b2ab)