Added String.init<T>(_: T) and marked it unavailable

Author: Harlan Haskins

stdlib/public/core/String.swift

@@ -1046,6 +1046,11 @@ extension String {
   public var uppercaseString: String {
     Builtin.unreachable()
   }
+
+  @available(*, unavailable, renamed: "init(describing:)")
+  public init<T>(_: T) {
+    Builtin.unreachable()
+  }
 }
 
 extension Sequence where Iterator.Element == String {

test/1_stdlib/StringDiagnostics_without_Foundation.swift

@@ -23,3 +23,9 @@ func testStringCollectionTypes(s: String) {
   acceptsBidirectionalCollection(s.characters)
   acceptsRandomAccessCollection(s.characters) // expected-error{{argument type 'String.CharacterView' does not conform to expected type 'RandomAccessCollection'}}
 }
+
+struct NotLosslessStringConvertible {}
+
+func testStringInitT() {
+  _ = String(NotLosslessStringConvertible()) // expected-error{{'init' has been renamed to 'init(describing:)}}
+}