relates to:

• SR-5506 Change name mangling of subscripts to no longer contain the string "subscript"

Issue Description:

Compiling the following code crashes in SILGen because the instance variable "`subscript`" and the subscript declaration have the same signature and their getters thus share the same mangled name.

struct Input {
}

struct Output {
}

class Foo {
  var `subscript`: (Input) -> Output {
    fatalError()
  }

  subscript(myarg: Input) -> Output {
    fatalError()
  }

  init() {
    fatalError()
  }
}