Fixup some test cases

Author: DougGregor

test/Serialization/macros.swift

@@ -13,5 +13,5 @@ func test(a: Int, b: Int) {
   // expected-error@-1{{external macro implementation type 'SomeModule.StringifyMacro' could not be found for macro 'publicStringify'; the type must be public and provided via '-load-plugin-library'}}
 
   _ = #internalStringify(a + b)
-  // expected-error@-1{{macro 'internalStringify' is undefined}}
+  // expected-error@-1{{no macro named 'internalStringify'}}
 }

test/StringProcessing/Parse/forward-slash-regex.swift

@@ -168,7 +168,7 @@ func testSubscript(_ x: S) {
   x[/x/]
   x[/x /]
   // expected-error@-1:9 {{expected expression after operator}}
-  // expected-error@-2 {{missing argument for parameter #1 in call}}
+  // expected-error@-2 {{missing argument for parameter #1 in subscript}}
 
   _ = x[/] / 2
 }

test/StringProcessing/Parse/regex_parse_error.swift

@@ -38,7 +38,7 @@ do {
   /#
   // expected-error@-2:7 {{unterminated regex literal}}
   // expected-error@-3:10 {{expected escape sequence}}
-  // expected-error@-3:4 {{expected expression}}
+  // expected-error@-3:4 {{expected a macro identifier}}
 }
 
 func foo<T>(_ x: T, _ y: T) {}