[tests] Update tests with modified diagnostics

Author: Jumhyn

test/Constraints/diagnostics.swift

@@ -1137,7 +1137,7 @@ func badTypes() {
 // rdar://34357545
 func unresolvedTypeExistential() -> Bool {
   return (Int.self==_{})
-  // expected-error@-1 {{'_' can only appear in a pattern or on the left side of an assignment}}
+  // expected-error@-1 {{type of expression is ambiguous without more context}}
 }
 
 do {

test/expr/expressions.swift

@@ -865,7 +865,7 @@ func r20802757(_ z: inout Int = &g20802757) { // expected-error {{cannot provide
   print(z)
 }
 
-_ = _.foo // expected-error {{'_' can only appear in a pattern or on the left side of an assignment}}
+_ = _.foo // expected-error {{could not infer type for placeholder}}
 
 // <rdar://problem/22211854> wrong arg list crashing sourcekit
 func r22211854() {

validation-test/Sema/SwiftUI/sr14213.swift

@@ -8,7 +8,7 @@ import SwiftUI
 struct Experiment: View {
   var body: some View {
     HStack { // expected-error {{generic parameter 'Content' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}}emacs
-      Slider(value: <#T##Binding<BinaryFloatingPoint>#>, in: <#T##ClosedRange<BinaryFloatingPoint>#>, label: <#T##() -> _#>) // expected-error 3 {{editor placeholder in source file}} expected-error {{expected type for function result}}
+      Slider(value: <#T##Binding<BinaryFloatingPoint>#>, in: <#T##ClosedRange<BinaryFloatingPoint>#>, label: <#T##() -> _#>) // expected-error 3 {{editor placeholder in source file}}
       // expected-error@-1 {{protocol 'BinaryFloatingPoint' as a type cannot conform to 'Comparable'}}
       // expected-note@-2 {{only concrete types such as structs, enums and classes can conform to protocols}}
     }