c++: Disable deprecated/unavailable diagnostics when creating thunks for methods...

c++: Disable deprecated/unavailable diagnostics when creating thunks for methods with such attributes [PR116636]

On the following testcase, we emit false positive warnings/errors about using
the deprecated or unavailable methods when creating thunks for them, even
when nothing (in the testcase so far) actually used those.

The following patch temporarily disables that diagnostics when creating
the thunks.

2024-09-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/116636
	* method.cc: Include decl.h.
	(use_thunk): Temporarily change deprecated_state to
	UNAVAILABLE_DEPRECATED_SUPPRESS.

	* g++.dg/warn/deprecated-19.C: New test.

(cherry picked from commit 4026d89d623e322920b052f7ac0d940ef267dc0f)