Add void cast to suppress -Wunused-member-variable on assert-only member

mlir/lib/Parser/Parser.cpp

@@ -1492,6 +1492,7 @@ public:
     }
 
     // Try to parse the region.
+    (void)isIsolatedFromAbove;
     assert((!enableNameShadowing || isIsolatedFromAbove) &&
            "name shadowing is only allowed on isolated regions");
     if (parser.parseRegion(region, regionArguments, enableNameShadowing))