Add a "Debug Build" flag to HermesInternal.getRuntimeProperties()

Summary:
This diff adds a "Debug Build" key to the RuntimeProperties to easily
determine whether the VM was built with `-DNDEBUG` or `-UNDEBUG`.

Reviewed By: tmikov

Differential Revision: D22004322

fbshipit-source-id: 9a12e681b758a95f9eebb5f7e1198aa66e3f131c

lib/VM/JSLib/HermesInternal.cpp

@@ -450,6 +450,24 @@ hermesInternalGetRuntimeProperties(void *, Runtime *runtime, NativeArgs args) {
     return ExecutionStatus::EXCEPTION;
   }
 
+  const char buildMode[] =
+#ifdef HERMES_SLOW_DEBUG
+      "SlowDebug"
+#elif !defined(NDEBUG)
+      "Debug"
+#else
+      "Release"
+#endif
+      ;
+  auto buildModeVal = StringPrimitive::create(
+      runtime, ASCIIRef(buildMode, sizeof(buildMode) - 1));
+  if (LLVM_UNLIKELY(
+          buildModeVal == ExecutionStatus::EXCEPTION ||
+          addProperty(runtime->makeHandle(*buildModeVal), "Build") ==
+              ExecutionStatus::EXCEPTION)) {
+    return ExecutionStatus::EXCEPTION;
+  }
+
   return resultHandle.getHermesValue();
 }