Fix parsing error on "unsafeAddress" and "unsafeMutableAddress" accessors

Our parser fails on

  public var pointee: Pointee {
    @_transparent unsafeAddress {
      return self
    }
  }

Which is invalid according to the Swift 5.3 official grammar (https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_variable-declaration), yet it is used in the Swift standard library:
https://github.com/apple/swift/blob/main/stdlib/public/core/UnsafePointer.swift#L241

These definitions in the Swift compiler might be relevant:
https://github.com/apple/swift/blob/main/include/swift/AST/AccessorKinds.def#L165
https://github.com/apple/swift/blob/main/include/swift/AST/AccessorKinds.def#L175