Use Builtin.unreachable() in unavailable functions.

Saves a bit of code size in the standard library by eliminating some
static strings and function calls.

rdar://problem/25767016

stdlib/public/core/StringCharacterView.swift

@@ -344,13 +344,13 @@ extension String.CharacterView {
   >(
     _ subRange: Range<Index>, with newElements: C
   ) {
-    fatalError("unavailable function can't be called")
+    Builtin.unreachable()
   }
     
   @available(*, unavailable, renamed: "append(contentsOf:)")
   public mutating func appendContentsOf<
     S : Sequence where S.Iterator.Element == Character
   >(_ newElements: S) {
-    fatalError("unavailable function can't be called")
+    Builtin.unreachable()
   }
 }