- In PTHLexer::Lex read all of the token data from PTH file before
  constructing the token.  The idea is to enhance locality.
- Do not use Read8/Read32 in PTHLexer::Lex.  Inline these operations manually.
- Change PTHManager::ReadIdentifierInfo() to PTHManager::GetIdentifierInfo().
  They are functionally the same except that PTHLexer::Lex() reads the
  persistent id.

These changes result in a 3.3% speedup for PTH on Cocoa.h (-Eonly).

llvm-svn: 61363