Suppress -Wunused-private-field warning in WasmTypeDefinition.h

Unreviewed stable branch commit.

This appears to be an LLVM bug, since the field is definitely used. The
bug does not occur on the main branch, so no sense in thinking too much
about it.

* Source/JavaScriptCore/wasm/WasmTypeDefinition.h:

Author: mcatanzaro

Source/JavaScriptCore/wasm/WasmTypeDefinition.h

@@ -773,7 +773,9 @@ class RTT_ALIGNMENT RTT : public ThreadSafeRefCounted<RTT> {
     // Payload starts past end of this object.
     const RTT** payload() { return static_cast<const RTT**>(static_cast<void*>(this + 1)); }
 
+IGNORE_WARNINGS_BEGIN("unused-private-field")
     RTTKind m_kind;
+IGNORE_WARNINGS_END
     DisplayCount m_displaySize;
 };