Fix for issue #858 (disallow incrementing/decrementing the value of enum)

Fix for issue #858. Updated the baselines associated with incrementing and decrementing enum types as well.  

This fix is a pretty simple but would it make more sense to add a new SyntaxKind for EnumAccess? I haven't looked too deeply into this but I just wonder if there are other places where we have to check IndexAccess and PropertyAccess to see if they really are accessing an Enum that a refactor like this would simplify.
Enums can merge with instantiated modules. This check appears to allow accesses to enums that are merged with modules. For instance:
```TS
module M {
    export var x;
}
enum M {
    y
}
var expr: string;
M["x"] = 1; // want to allow this one
M["y"] = 1; // want to disallow this one
M[expr] = 1; // what to do for this one?
```
Right now these would all be allowed, but that's likely not the intention. I would say try to get the symbol for the IndexedAccess. If you got one, check if it's an EnumMember (just like the PropertyAccess case above). If not, then if the IndexedAccess.object is *at least* an enum, give the error. This last part could be too strict, so I'm open to being more lenient there.