[NFC] Add a test for @_implements with non-identifier type argument

This also ensures that the new Swift parser can handle it too.

Author: AnthonyLatsis

test/attr/attr_implements.swift

@@ -3,6 +3,7 @@
 
 protocol P {
   func f0() -> Int;
+  func f1() -> Int
   func f(x:Int, y:Int) -> Int;
 }
 
@@ -19,6 +20,12 @@ struct S : P, Q, Equatable {
     return 10
   }
 
+  // Test that we can handle a non-identifier type that canonicalizes
+  // to a protocol.
+  @_implements((P), f1())
+  func g1() -> Int { 11 }
+
+
   // Test that it's possible to implement two different protocols with the
   // same-named requirements.
   @_implements(P, f(x:y:))