[cindex.py] Add cache for CompletionChunk spellings

Most of the CompletionChunks represent braces, colons or other one
character spellings. There is no need to call libclang, to figure out
how to write a colon. Instead we use an internal cache to retrieve the
correct spelling. As function calls from python are very expensive and
this is a performance critical part of auto completion this patch makes
formatting of auto completion results a lot faster.

Formatting time changes from 0.57 to 0.45 seconds

llvm-svn: 172901