Allow the preprocessor to cache the lexed tokens, so that we can do efficient lookahe...

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