Tweak key path tests for swift-4.2-branch.

Author: rudkx

test/Constraints/keypath.swift

@@ -6,16 +6,16 @@ struct S {
   init() {
     let _: WritableKeyPath<S, Int> = \.i // no error for Swift 3/4
 
-    S()[keyPath: \.i] = 1
+    S()[keyPath: \S.i] = 1
     // expected-error@-1 {{cannot assign to immutable expression}}
   }
 }
 
 func test() {
   let _: WritableKeyPath<C, Int> = \.i // no error for Swift 3/4
 
-  C()[keyPath: \.i] = 1   // warning on write with literal keypath
+  C()[keyPath: \C.i] = 1   // warning on write with literal keypath
   // expected-warning@-1 {{forming a writable keypath to property}}
 
-  let _ = C()[keyPath: \.i] // no warning for a read
+  let _ = C()[keyPath: \C.i] // no warning for a read
 }

test/Constraints/keypath_swift_5.swift

@@ -6,16 +6,16 @@ struct S {
   init() {
     let _: WritableKeyPath<S, Int> = \.i // expected-error {{type of expression is ambiguous without more context}}
 
-    S()[keyPath: \.i] = 1
+    S()[keyPath: \S.i] = 1
     // expected-error@-1 {{cannot assign to immutable expression}}
   }
 }
 
 func test() {
   let _: WritableKeyPath<C, Int> = \.i // expected-error {{type of expression is ambiguous without more context}}
 
-  C()[keyPath: \.i] = 1
+  C()[keyPath: \C.i] = 1
   // expected-error@-1 {{cannot assign to immutable expression}}
 
-  let _ = C()[keyPath: \.i] // no warning for a read
+  let _ = C()[keyPath: \C.i] // no warning for a read
 }