deps: always define V8_NODISCARD as no-op

It's causing compiler errors with some classes on Xcode 11
and the attribute should have no runtime effect.

PR-URL: nodejs/node#54077
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

common.gypi

@@ -36,7 +36,7 @@
 
     # Reset this number to 0 on major V8 upgrades.
     # Increment by one for each non-official patch applied to deps/v8.
-    'v8_embedder_string': '-node.5',
+    'v8_embedder_string': '-node.6',
 
     ##### V8 defaults for Node.js #####
 

deps/v8/include/v8config.h

@@ -677,7 +677,7 @@ path. Add it with -I<path> to the command line
 //   V8_NODISCARD Foo() { ... };
 // [[nodiscard]] comes in C++17 but supported in clang with -std >= c++11.
 #if V8_HAS_CPP_ATTRIBUTE_NODISCARD
-#define V8_NODISCARD [[nodiscard]]
+#define V8_NODISCARD
 #else
 #define V8_NODISCARD /* NOT SUPPORTED */
 #endif