Silence `-Wunused-private-field` warning on isIsolatedFromAbove.

[csigg]

Summary

NDEBUG builds currently warn because it's only used inside an assert.

mlir/lib/Parser/Parser.cpp

 class CustomOpAsmParser : public OpAsmParser {
 public:
   CustomOpAsmParser(
       SMLoc nameLoc, ArrayRef<OperationParser::ResultRecord> resultIDs,
       function_ref<ParseResult(OpAsmParser &, OperationState &)> parseAssembly,
       bool isIsolatedFromAbove, StringRef opName, OperationParser &parser)
       : nameLoc(nameLoc), resultIDs(resultIDs), parseAssembly(parseAssembly),
         isIsolatedFromAbove(isIsolatedFromAbove), opName(opName),
-        parser(parser) {}
+        parser(parser) {
+    (void)isIsolatedFromAbove; // Only used in assert, silence unused warning.
+  }
 
   /// Parse an instance of the operation described by 'opDefinition' into the
   /// provided operation state.
   ParseResult parseOperation(OperationState &opState) {
     if (parseAssembly(*this, opState))
       return failure();
     // Verify that the parsed attributes does not have duplicate attributes.
     // This can happen if an attribute set during parsing is also specified in