Allow the preprocessor to cache the lexed tokens, so that we can do efficient lookahead and backtracking.

1) New public methods added:
  -EnableBacktrackAtThisPos
  -DisableBacktrack
  -Backtrack
  -isBacktrackEnabled

2) LookAhead() implementation is replaced with a more efficient one.
3) LookNext() is removed.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@54611 91177308-0d34-0410-b5e6-96231b3b80d8