PTH:

PTH:
- Embed 'eom' tokens in PTH file.
- Use embedded 'eom' tokens to not lazily generate them in the PTHLexer.
  This means that PTHLexer can always advance to the next token after
  reading a token (instead of buffering tokens using a copy).
- Moved logic of 'ReadToken' into Lex.  GetToken & ReadToken no longer exist.
- These changes result in a 3.3% speedup (-Eonly) on Cocoa.h.
- The code is a little gross.  Many cleanups are possible and should be done.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@61360 91177308-0d34-0410-b5e6-96231b3b80d8