[CodeComplete] Don't emit 'override' in protocol extension

[rintaro]

• Don't emit any inherited decls in protocol declarations.
• Don't emit override keyword in protocol extensions.

Previously:

protocol BaseP {
  init(base: Int)
  func baseFunc()
  var baseVal: Int { get }
}

protocol TestP : BaseP {
   #^PROTO^#
}
extension TestP {
  #^PROTO_EXT^#
}

For both #^PROTO^# and #^PROTO_EXT^#:

Begin completions, 3 items
Decl[Constructor]/Super:            override init(base: Int) {|}; name=init(base: Int)
Decl[InstanceMethod]/Super:         override func baseFunc() {|}; name=baseFunc()
Decl[InstanceVar]/Super:            override var baseVal: Int; name=baseVal: Int
End completions

We don't want any for #^PROTO^#.
override is invalid for #^PROTO_EXT^#.

CC/ @benlangmuir

[rintaro]

@swift-ci Please smoke test

[benlangmuir]

LGTM