[torque] Add definition for constexpr member

TBR=tebbi@chromium.org

Bug: v8:9239
Change-Id: I9634001a4b6a46680175b9dd04075a313788514c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1609544Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61452}

src/torque/types.cc

@@ -289,6 +289,8 @@ std::string StructType::ToExplicitString() const {
   return result.str();
 }
 
+constexpr ClassFlags ClassType::kInternalFlags;
+
 ClassType::ClassType(const Type* parent, Namespace* nspace,
                      const std::string& name, ClassFlags flags,
                      const std::string& generates, const ClassDeclaration* decl,