This change makes us treat it exactly as we do 'init'. We don't allow renaming the base name,
and don't fail if the basename doesn't match for calls.
Also:
- explicit init calls/references like `MyType.init(42)` are now reported with
'init' as a keywordBase range, rather than nothing.
- cursor info no longer reports rename as available on init/callAsFunction
calls without arguments, as there's nothing to rename in that case.
- Improved detection of when a referenced function is a call (rather than
reference) across syntactic rename, cursor-info, and indexing.
Resolves rdar://problem/60340429
Adds support for renaming subscripts with external names, e.g.
subscript(x y: Int), and introduces a noncollapsible parameter name range for
subscript parameters, since these shouldn't be collapsed with an argument label
of the same name as function parameter names are.
