[cindex.py] Replace CachedProperty with our own implementation

This is a very performance critical point for auto completion. The manual
implementation gives a large speedup. As it does not complicate the code a lot,
I figured it is worth the change. If anybody understands why the CachedProperty
is here so much slower, I am very interested in working on an improvement of
CachedProperty.

Formatting time changes from 0.72 to 0.57 seconds.

llvm-svn: 172900