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/ScalarType.h b/ScalarType.h
index 80bc9a0..80f8159 100644
--- a/ScalarType.h
+++ b/ScalarType.h
@@ -44,6 +44,8 @@
     bool isElidableType() const override;
     const ScalarType *resolveToScalarType() const override;
 
+    bool canCheckEquality() const override;
+
     std::string typeName() const override;
     bool isValidEnumStorageType() const;