- Move static function IsNonPragmaNonMacroLexer into Preprocessor.h.

- Add variants of IsNonPragmaNonMacroLexer to accept an IncludeMacroStack entry
  (simplifies some uses).
- Use IsNonPragmaNonMacroLexer in Preprocessor::LookupFile.
- Add 'FileID' to PreprocessorLexer, and have Preprocessor query this fileid
  when looking up the FileEntry for a file

Performance testing of -Eonly on Cocoa.h shows no performance regression because
of this patch.

llvm-svn: 59666