Refactor and micro-optimize ConsumeToken()

1) Introduce TryConsumeToken() to handle the common test-and-consume pattern.
   This brings about readability improvements in the parser and optimizes to avoid
   redundant checks in the common case.

2) Eliminate the ConsumeCodeCompletionTok special case from ConsumeToken(). This
   was used by only one caller which has been switched over to the more
   appropriate ConsumeCodeCompletionToken() function.

llvm-svn: 197497