fixup! [Sema] Add fix-it for property in protocol

Author: mzp

test/decl/var/properties.swift

@@ -881,6 +881,9 @@ protocol ProtocolWillSetDidSet3 {
 protocol ProtocolWillSetDidSet4 {
   var a: Int { didSet willSet } // expected-error {{property in protocol must have explicit { get } or { get set } specifier}} {{14-32={ get <#set#> \}}} expected-error 2 {{expected get or set in a protocol property}}
 }
+protocol ProtocolWillSetDidSet5 {
+  let a: Int { didSet willSet }  // expected-error {{property in protocol must have explicit { get } or { get set } specifier}} {{14-32={ get <#set#> \}}} {{none}} expected-error 2 {{expected get or set in a protocol property}} expected-error {{'let' declarations cannot be computed properties}} {{3-6=var}}
+}
 
 var globalDidsetWillSet: Int {  // expected-error {{non-member observing properties require an initializer}}
   didSet {}