Add autocomplete complete support for enum atoms
Summary:
Given
```
function f<T>(<<__Atom>> HH\MemberOf<EE, T> $m): void {}
```
This is meant trigger autocomplete on `f#` with nearly the same results as `EE::`. I just send back the constants of the enum class, ignoring the `::class` constant which is used for classname (not relevant to atoms).
The enum atom rewriting works out nicely, because when we see `f#AUTO332` the parser guarantees that we're in an `Aast.Call` node, so I can just look at `Call` nodes where the first argument is an `EnumAtom`.
Reviewed By: Wilfred
Differential Revision:
D27145618
fbshipit-source-id:
f7a68638c98e250f232eb804233407030d31c7b5