Avoid decoding already-decoded strings from cindex.

Recent versions of clang.cindex include [code][1] that converts
the internal byte strings to python str for you.

  [1]: https://github.com/llvm-mirror/clang/blob/master/bindings/python/clang/cindex.py#L72

tools/mkdoc.py

@@ -59,7 +59,7 @@ job_semaphore = Semaphore(job_count)
 output = []
 
 def d(s):
-    return s.decode('utf8')
+    return s if isinstance(s, str) else s.decode('utf8')
 
 
 def sanitize_name(name):