Add operator== and != for some structs.

operator== and != are defined for type T where T is
one of the following:
* scalars
* strings
* enums
* U[] where U is in this list
* vec<U> where U is in this list
* structs that contain only types in this list.

If a struct contains a handle, an MQDescriptor,
or a union, for example, then == and != is not generated.

Bug: 32834072

Test: hidl_test
Change-Id: Icac73a14f81274ee6592e2b8fadf26d629151d9f
diff --git a/StringType.h b/StringType.h
index 085ec11..3c1e10a 100644
--- a/StringType.h
+++ b/StringType.h
@@ -29,6 +29,8 @@
 
     bool isString() const override;
 
+    bool canCheckEquality() const override;
+
     std::string getCppType(
             StorageMode mode,
             bool specifyNamespaces) const override;