Mark some more conversion functions as @readonly.

On -Ounchecked we are now able to zap this expression (from Richards):
  UnicodeScalar(UInt32(2)+"0".value)

However, on -O we still check for overflow and actually need to construct the string.



Swift SVN r20856

stdlib/core/String.swift

@@ -62,6 +62,7 @@ extension String {
 }
 
 extension String : _BuiltinExtendedGraphemeClusterLiteralConvertible {
+  @semantics("readonly")
   public
   static func _convertFromBuiltinExtendedGraphemeClusterLiteral(
     start: Builtin.RawPointer,