src: add rudimentary Promise support

This patch allows llnode to list Promise objects with findjsobjects,
findjsinstances and findrefs. We can investigate more fancy Promise
features in the future (such as listing handlers, Promise status, etc.).

For Node.js v10.x, since we don't have the JS_PROMISE type as postmortem
metadata, we assume JS_PROMISE is the next type after
JS_MESSAGE_OBJECT_TYPE. This is a safe assumption for Node.js v10.x,
v12.x has the JS_PROMISE type, and v8.x is not supported anymore.

```console
$ git log v10.0.0..v10.17.0 -L :InstanceType:deps/v8/src/objects.h | grep -C2 "JS_PROMISE"
   JS_MAP_VALUE_ITERATOR_TYPE,
   JS_MESSAGE_OBJECT_TYPE,
   JS_PROMISE_TYPE,
   JS_REGEXP_TYPE,
   JS_REGEXP_STRING_ITERATOR_TYPE,
--
   JS_MAP_VALUE_ITERATOR_TYPE,
   JS_MESSAGE_OBJECT_TYPE,
   JS_PROMISE_TYPE,
   JS_REGEXP_TYPE,
+  JS_REGEXP_STRING_ITERATOR_TYPE,
```

Author: mmarchini

src/llv8-constants.cc

@@ -518,6 +518,13 @@ void Types::Load() {
   kLastContextType = LoadConstant("LastContextType");
 
   kJSErrorType = LoadConstant("type_JSError__JS_ERROR_TYPE");
+  kJSPromiseType = LoadConstant("type_JSPromise__JS_PROMISE_TYPE");
+  if (kJSPromiseType == -1) {
+    // NOTE(mmarchini): On Node.js v10.x, JS_PROMISE always comes after
+    // JS_MESSAGE_OBJECT_TYPE in the InstanceType enum.
+    kJSPromiseType =
+        LoadConstant("type_JSMessageObject__JS_MESSAGE_OBJECT_TYPE") + 1;
+  }
   kHeapNumberType = LoadConstant("type_HeapNumber__HEAP_NUMBER_TYPE");
   kMapType = LoadConstant("type_Map__MAP_TYPE");
   kGlobalObjectType =

src/llv8-constants.h

@@ -507,6 +507,7 @@ class Types : public Module {
   int64_t kLastContextType;
 
   int64_t kJSErrorType;
+  int64_t kJSPromiseType;
   int64_t kHeapNumberType;
   int64_t kMapType;
   int64_t kGlobalObjectType;

src/llv8-inl.h

@@ -314,6 +314,7 @@ inline bool JSObject::IsObjectType(LLV8* v8, int64_t type) {
   return type == v8->types()->kJSObjectType ||
          type == v8->types()->kJSAPIObjectType ||
          type == v8->types()->kJSErrorType ||
+         type == v8->types()->kJSPromiseType ||
          type == v8->types()->kJSSpecialAPIObjectType;
 }