[test] fix diagnostics tests for String-from-C-string inits

Author: glessard

test/stdlib/StringAPICStringDiagnostics.swift

@@ -6,7 +6,9 @@ func checkStringOverloadCompilationDiagnostics() {
 
   _ = String(cString: "string") // expected-warning {{'init(cString:)' is deprecated: Use a copy of the String argument}}
 
-  _ = String(validatingCString: "string") // expected-warning {{init(validatingCString:)' is deprecated: Use a copy of the String argument}}
+  _ = String(validatingUTF8: "string") // expected-warning {{init(validatingUTF8:)' is deprecated: Use a copy of the String argument}}
+
+  _ = String(validatingCString: "string") // expected-warning {{'init(validatingCString:)' is deprecated: Use a copy of the String argument}}
 
   _ = String.decodeCString("string", as: Unicode.UTF8.self) // expected-warning {{'decodeCString(_:as:repairingInvalidCodeUnits:)' is deprecated: Use a copy of the String argument}}
 
@@ -23,7 +25,9 @@ func checkInoutConversionOverloadCompilationDiagnostics() {
 
   _ = String(cString: &c) // expected-warning {{'init(cString:)' is deprecated: Use String(_ scalar: Unicode.Scalar)}}
 
-  _ = String(validatingCString: &c) // expected-warning {{init(validatingCString:)' is deprecated: Use String(_ scalar: Unicode.Scalar)}}
+  _ = String(validatingUTF8: &c) // expected-warning {{init(validatingUTF8:)' is deprecated: Use String(_ scalar: Unicode.Scalar)}}
+
+  _ = String(validatingCString: &c) // expected-warning {{'init(validatingCString:)' is deprecated: Use String(_ scalar: Unicode.Scalar)}}
 
   var u = Unicode.UTF8.CodeUnit.zero