c++: Implement CWG 2654 - Un-deprecation of compound volatile assignments

The following patch implements CWG 2654.

2022-11-16  Jakub Jelinek  <jakub@redhat.com>

	* typeck.cc (cp_build_modify_expr): Implement CWG 2654
	- Un-deprecation of compound volatile assignments.  Remove
	-Wvolatile warning about compound volatile assignments.

	* g++.dg/cpp2a/volatile1.C (fn2, fn3, racoon): Adjust expected
	diagnostics.
	* g++.dg/cpp2a/volatile3.C (fn2, fn3, racoon): Likewise.
	* g++.dg/cpp2a/volatile5.C (f): Likewise.
	* g++.dg/ext/vector25.C (foo): Don't expect a warning.
	* g++.dg/cpp1y/new1.C (test_unused): Likewise.