Tests: Test conformance checker with witnesses in unavailable extensions.

A protocol conformance witness must be as available as its requirement. In
Swift 6.1 and earlier, the conformance checker failed to note that witnesses in
unavailable extensions are unavailable. That bug was fixed by a previous
change, but there was no test case covering it so the difference in behavior
was not acknowledged.

Related to rdar://143466010.

Author: tshortli

test/decl/protocol/req/unavailable.swift

@@ -26,7 +26,7 @@ class Bar : NonObjCProto { // expected-error {{type 'Bar' does not conform to pr
 
 // Complain about unavailable witnesses (error in Swift 4, warning in Swift 3)
 protocol P {
-  func foo(bar: Foo) // expected-note 2 {{requirement 'foo(bar:)' declared here}}
+  func foo(bar: Foo) // expected-note 3 {{requirement 'foo(bar:)' declared here}}
 }
 
 struct ConformsToP : P { // expected-error{{type 'ConformsToP' does not conform to protocol 'P'}}
@@ -64,6 +64,14 @@ extension ConformsToP5: P {
   func foo(bar: Foo) { }
 }
 
+struct ConformsToP6: P {} // expected-error{{type 'ConformsToP6' does not conform to protocol 'P'}}
+// expected-error@-1 {{unavailable instance method 'foo(bar:)' was used to satisfy a requirement of protocol 'P'}}
+
+@available(*, unavailable)
+extension ConformsToP6 {
+  func foo(bar: Foo) { } // expected-note {{'foo(bar:)' declared here}}
+}
+
 @available(*, unavailable)
 enum UnavailableEnum {
   struct ConformsToP6: P {