Teach typo correction about various language keywords. We can't

generally recover from typos in keywords (since we would effectively
have to mangle the token stream). However, there are still benefits to
typo-correcting with keywords:
  - We don't make stupid suggestions when the user typed something
  that is similar to a keyword. 
  - We can suggest the keyword in a diagnostic (did you mean
  "static_cast"?), even if we can't recover and therefore don't have
  a fix-it.

llvm-svn: 101274