Clarify function_type_argument_label error msg

[SR-2451]
https://bugs.swift.org/browse/SR-2451

Author: nathanhosselton

include/swift/AST/DiagnosticsParse.def

@@ -620,7 +620,7 @@ ERROR(expected_type_before_arrow,none,
 ERROR(expected_type_after_arrow,none,
       "expected type after '->'", ())
 ERROR(function_type_argument_label,none,
-      "function types cannot have argument label %0; use '_' instead",
+      "function types cannot have argument labels; use '_' before %0",
       (Identifier))
 
 // Enum Types

test/type/function/labels.swift

@@ -1,27 +1,27 @@
 // RUN: %target-swift-frontend -module-name TestModule -parse -verify %s
 
 // Function type with various forms of argument label.
-typealias Function1 = (a: Int,   // expected-error{{function types cannot have argument label 'a'; use '_' instead}}{{24-24=_ }}
+typealias Function1 = (a: Int,   // expected-error{{function types cannot have argument labels; use '_' before 'a'}}{{24-24=_ }}
                        _ b: Int, // okay
-                       c d: Int, // expected-error{{function types cannot have argument label 'c'; use '_' instead}}{{24-25=_}}
-                       e _: Int, // expected-error{{function types cannot have argument label 'e'; use '_' instead}}{{24-29=}}
+                       c d: Int, // expected-error{{function types cannot have argument labels; use '_' before 'c'}}{{24-25=_}}
+                       e _: Int, // expected-error{{function types cannot have argument labels; use '_' before 'e'}}{{24-29=}}
                        _: Int,   // okay
                        Int)      // okay
     -> Int
 
 // Throwing versions.
-typealias Function2 = (a: Int,   // expected-error{{function types cannot have argument label 'a'; use '_' instead}}{{24-24=_ }}
+typealias Function2 = (a: Int,   // expected-error{{function types cannot have argument labels; use '_' before 'a'}}{{24-24=_ }}
                        _ b: Int, // okay
-                       c d: Int, // expected-error{{function types cannot have argument label 'c'; use '_' instead}}{{24-25=_}}
-                       e _: Int, // expected-error{{function types cannot have argument label 'e'; use '_' instead}}{{24-29=}}
+                       c d: Int, // expected-error{{function types cannot have argument labels; use '_' before 'c'}}{{24-25=_}}
+                       e _: Int, // expected-error{{function types cannot have argument labels; use '_' before 'e'}}{{24-29=}}
                        _: Int,   // okay
                        Int)      // okay
     throws -> Int
 
-typealias Function3 = (a: Int,   // expected-error{{function types cannot have argument label 'a'; use '_' instead}}{{24-24=_ }}
+typealias Function3 = (a: Int,   // expected-error{{function types cannot have argument labels; use '_' before 'a'}}{{24-24=_ }}
                        _ b: Int, // okay
-                       c d: Int, // expected-error{{function types cannot have argument label 'c'; use '_' instead}}{{24-25=_}}
-                       e _: Int, // expected-error{{function types cannot have argument label 'e'; use '_' instead}}{{24-29=}}
+                       c d: Int, // expected-error{{function types cannot have argument labels; use '_' before 'c'}}{{24-25=_}}
+                       e _: Int, // expected-error{{function types cannot have argument labels; use '_' before 'e'}}{{24-29=}}
                        _: Int,   // okay
                        Int)      // okay
     rethrows -> Int              // expected-error{{only function declarations may be marked 'rethrows'}}