Made declaring the underlying type of an enum mandatory.

This is a breaking change, anyone having schema files with enums
that do not specify a type will get a specialized error:

must specify the underlying integer type for this
enum (e.g. ': short', which was the default).

All of the samples and docs already had a type specified,
so hopefully this will affect very few people.

Bug: 15777205
Change-Id: I9b8d7c0827867f7efb6c217346db7e402695eff0
Tested: on Windows