c++: Fix bogus -Wparentheses warning [PR95344]

Since r267272, which added location wrappers, cp_fold loses
TREE_NO_WARNING on a MODIFY_EXPR that finish_parenthesized_expr set, and
that results in a bogus -Wparentheses warning.

I.e., previously we had "b = 1" but now we have "VIEW_CONVERT_EXPR<bool>(b) = 1"
and cp_fold_maybe_rvalue folds away the location wrapper and so we do
2718             x = fold_build2_loc (loc, code, TREE_TYPE (x), op0, op1);
in cp_fold and the flag is lost.

	PR c++/95344
	* cp-gimplify.c (cp_fold) <case MODIFY_EXPR>: Don't set
	TREE_THIS_VOLATILE here.
	(cp_fold): Set it here along with TREE_NO_WARNING.

	* c-c++-common/Wparentheses-2.c: New test.

gcc/cp/cp-gimplify.c

@@ -2675,8 +2675,6 @@ cp_fold (tree x)
 	  else
 	    x = org_x;
 	}
-      if (code == MODIFY_EXPR && TREE_CODE (x) == MODIFY_EXPR)
-	TREE_THIS_VOLATILE (x) = TREE_THIS_VOLATILE (org_x);
 
       break;
 
@@ -2932,6 +2930,12 @@ cp_fold (tree x)
       return org_x;
     }
 
+  if (EXPR_P (x) && TREE_CODE (x) == code)
+    {
+      TREE_THIS_VOLATILE (x) = TREE_THIS_VOLATILE (org_x);
+      TREE_NO_WARNING (x) = TREE_NO_WARNING (org_x);
+    }
+
   fold_cache->put (org_x, x);
   /* Prevent that we try to fold an already folded result again.  */
   if (x != org_x)

gcc/testsuite/c-c++-common/Wparentheses-2.c

+// PR c++/95344 - bogus -Wparentheses warning.
+// { dg-do compile }
+// { dg-options "-Wparentheses" }
+
+#ifndef __cplusplus
+# define bool _Bool
+# define true 1
+# define false 0
+#endif
+
+void
+f (int i)
+{
+  bool b = false;
+  if (i == 99 ? (b = true) : false) // { dg-bogus "suggest parentheses" }
+    {
+    }
+}