- Add 'CurPPLexer' to Preprocessor to keep track of the current

  PreprocessorLexer, which will either be a 'Lexer' or 'PTHLexer'.
- Added stub field 'CurPTHLexer' to keep track of the current PTHLexer.
- Modified IncludeStackInfo to track both the current PTHLexer and
  current PreprocessorLexer.

llvm-svn: 59472