stdlib: unavailable String APIs: use Swift 2.x APIs without first argument labels (#3022)

Author: gribozavr

stdlib/public/core/UnavailableStringAPIs.swift.gyb

@@ -131,19 +131,19 @@ extension ${Index} {
   @available(
     *, unavailable,
     message: "To advance an index by n steps call 'index(_:offsetBy:)' on the ${View} instance that produced the index.")
-  public func advancedBy(n: ${Distance}) -> ${Index} {
+  public func advancedBy(_ n: ${Distance}) -> ${Index} {
     Builtin.unreachable()
   }
   @available(
     *, unavailable,
     message: "To advance an index by n steps stopping at a given limit call 'index(_:offsetBy:limitedBy:)' on ${View} instance that produced the index.  Note that the Swift 3 API returns 'nil' when trying to advance past the limit; the Swift 2 API returned the limit.")
-  public func advancedBy(n: ${Distance}, limit: ${Index}) -> ${Index} {
+  public func advancedBy(_ n: ${Distance}, limit: ${Index}) -> ${Index} {
     Builtin.unreachable()
   }
   @available(
     *, unavailable,
     message: "To find the distance between two indices call 'distance(from:to:)' on the ${View} instance that produced the index.")
-  public func distanceTo(end: ${Index}) -> ${Distance} {
+  public func distanceTo(_ end: ${Index}) -> ${Distance} {
     Builtin.unreachable()
   }
 }