Follow convention for struct names in special_names

Follow convention for struct names in special_names

Summary: As edwinsmith pointed out on {D33820141 (https://github.com/facebook/hhvm/commit/355891dcbf5964c6a075e0b9d1ebbe6e5c4db1f2)}, the convention for initialisms in struct names is to use PascalCase and treat the initialism as a word. The field names in this file also break convention, but since they will actually appear in typing logic, I figure there might be some value to keeping their casing the same as naming_special_names.ml for now. We can circle back and change them to snake_case later. Since these struct names won't appear in typing logic, there's no reason to keep the unidiomatic names, so let's fix them now.

Reviewed By: edwinsmith

Differential Revision: D33822374

fbshipit-source-id: ad623b5ab59b37b539316aa5a16de3121475d495