Issue Description:

There seems to be no way to deprecate a member and retarget its invocations to a subscript. The `renamed:` argument of a deprecation doesn't accept a reference to a subscript member.

I expected something like this would work:

struct Foo {
  subscript(_ index: Int) -> String { "hello" }

  @available(*, deprecated, renamed: "subscript(_:)")
  subscript(offset offset: Int) -> String { "hello" }
}

Unfortunately, the resulting fix-it changes `self[42]` into `self.subscript`, which loses information, and doesn't even compile.