[iwyu] Fix failing header include checks for gcc build

Bug: v8:9290, v8:7490, v8:9183
Change-Id: Iac5cdf0d96c1fdbb7e6461ea435c1ff32c6a701e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1627543
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61821}

src/handles/handles.h

@@ -111,9 +111,14 @@ class Handle final : public HandleBase {
     T object_;
   };
 
-  V8_INLINE explicit Handle(Address* location = nullptr)
-      : HandleBase(location) {
-    // Type check:
+  V8_INLINE explicit Handle() : HandleBase(nullptr) {
+    // Skip static type check in order to allow Handle<XXX>::null() as default
+    // parameter values in non-inl header files without requiring full
+    // definition of type XXX.
+  }
+
+  V8_INLINE explicit Handle(Address* location) : HandleBase(location) {
+    // This static type check also fails for forward class declarations.
     static_assert(std::is_convertible<T*, Object*>::value,
                   "static type violation");
     // TODO(jkummerow): Runtime type check here as a SLOW_DCHECK?

src/handles/maybe-handles-inl.h

@@ -12,14 +12,11 @@
 
 namespace v8 {
 namespace internal {
+
 template <typename T>
 MaybeHandle<T>::MaybeHandle(T object, Isolate* isolate)
     : MaybeHandle(handle(object, isolate)) {}
 
-MaybeObjectHandle::MaybeObjectHandle()
-    : reference_type_(HeapObjectReferenceType::STRONG),
-      handle_(Handle<Object>::null()) {}
-
 MaybeObjectHandle::MaybeObjectHandle(MaybeObject object, Isolate* isolate) {
   HeapObject heap_object;
   DCHECK(!object->IsCleared());

src/handles/maybe-handles.h

@@ -81,7 +81,8 @@ class MaybeHandle final {
 // while the MaybeObjectHandle is alive.
 class MaybeObjectHandle {
  public:
-  inline MaybeObjectHandle();
+  inline MaybeObjectHandle()
+      : reference_type_(HeapObjectReferenceType::STRONG) {}
   inline MaybeObjectHandle(MaybeObject object, Isolate* isolate);
   inline MaybeObjectHandle(Object object, Isolate* isolate);
   inline explicit MaybeObjectHandle(Handle<Object> object);