1, restrict use of LiteralBuffers to the tokens that actually need it.
  - E.g., previously the Token::FUNCTION would have a literal buffer
    containing "function", which was never actually used.
  - This eliminates copies of the string data for every call to
    PeekAhead or SetBookmark.
2, document & enforce the "secret" Scanner API contract w/ DCHECK
  - Document & check the correspondence of token value and literal buffer.
  - Document & check preconditions for calling PeekAhead, ScanRegExp*,
    ScanTemplate*.

BUG=v8:4947

Review-Url: https://codereview.chromium.org/2240513003
Cr-Commit-Position: refs/heads/master@{#38677}