cmGeneratedFileStream: Add support for a temporary alternate encoding

The Ninja generator needs to be able to write some file content in a
different encoding than the rest of the file.  Add a method to do this.

Source/cmGeneratedFileStream.h

@@ -147,6 +147,12 @@ public:
    */
   void SetTempExt(std::string const& ext);
 
+  /**
+   * Write a specific string using an alternate encoding.
+   * Afterward, the original encoding is restored.
+   */
+  void WriteAltEncoding(std::string const& data, Encoding encoding);
+
   /**
    * Writes the given string directly to the file without changing the
    * encoding.