c++: Implement P2327R1 - De-deprecating volatile compound operations

c++: Implement P2327R1 - De-deprecating volatile compound operations

From what I can see, this has been voted in as a DR and as it means
we warn less often than before in -std={gnu,c}++2{0,3} modes or with
-Wvolatile, I wonder if it shouldn't be backported to affected release
branches as well.

2022-08-16  Jakub Jelinek  <jakub@redhat.com>

	* typeck.cc (cp_build_modify_expr): Implement
	P2327R1 - De-deprecating volatile compound operations.  Don't warn
	for |=, &= or ^= with volatile lhs.
	* expr.cc (mark_use) <case MODIFY_EXPR>: Adjust warning wording,
	leave out simple.

	* g++.dg/cpp2a/volatile1.C: Adjust for de-deprecation of volatile
	compound |=, &= and ^= operations.
	* g++.dg/cpp2a/volatile3.C: Likewise.
	* g++.dg/cpp2a/volatile5.C: Likewise.