[SourceKit] Add structure node for SubscriptExpr
Arguments in `SubscriptExpr` are visited since the recent `ArgumentList`
refactoring, but were being added to the containing `CallExpr`. Add a
node for the `SubscriptExpr` itself so that its argument is added there
instead of the `CallExpr`.
Also remove `key.nameoffset` and `key.namelength` from the response when
both are 0 to match the rest of the offsets and lengths.
Resolves rdar://85412164.
