Add KeyValuePairs typealias as prep for 5.0 rename (#17711)

Author: airspeedswift

stdlib/public/core/Mirror.swift

@@ -720,6 +720,12 @@ public struct DictionaryLiteral<Key, Value> : ExpressibleByDictionaryLiteral {
   internal let _elements: [(Key, Value)]
 }
 
+// NOTE: This will become the new name in Swift 5.0. This typealias is being
+// introduced in 4.2 in order to allow code to be migrated to the new name
+// using the old compiler, enabling warning-free compilation with both  the
+// old and new compiler. This is a swift-4.2-branch only change.
+public typealias KeyValuePairs<Key,Value> = DictionaryLiteral<Key,Value>
+
 /// `Collection` conformance that allows `DictionaryLiteral` to
 /// interoperate with the rest of the standard library.
 extension DictionaryLiteral : RandomAccessCollection {

test/stdlib/DictionaryLiteral.swift

@@ -51,3 +51,7 @@ expectType(DictionaryLiteral<String, NSString>.self, &stringNSStringLet)
 
 var hetero: DictionaryLiteral = ["a": 1 as NSNumber, "b": "Foo" as NSString]
 expectType(DictionaryLiteral<String, NSObject>.self, &hetero)
+
+let aliased: KeyValuePairs<String,String> = strings
+expectType(DictionaryLiteral<String, String>.self, &strings)
+