Refactor `addSyntacticRenameRanges`, adding comments to make it easier to follow and remove its dependency on the `IsFunctionLike` parameter in `RenameLoc`.
This allows building sourcekitd and swift-refactor with `SWIFT_BUILD_SWIFT_SYNTAX=NO`. In these builds, the `relatedidents` and `find-syntactic-rename-ranges` requests will always return an error.
I will remove the `syntactic-rename` refactoring action in a follow-up commit. Clients should always ask for rename ranges and perform the rename by themselves instead of asking for a renamed file.
This converts the instances of the pattern for which we have a proper
substitution in lit. This will make it easier to replace it
appropriately with Windows equivalents.
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.
