When Parsing JSON, it will read enums either as int values, identifiers
specific to the enum type, or strings containing those identifiers.

When generating text, it will output enum identifiers by default
(this can be turned off in favor of integers, like before).

Change-Id: If28b0a1f8f27de79aff3e626f40c0c0b271c325a
Tested: on Windows and Linux
Bug: 16214968