PTH:

- Added stub PTHLexer::getSpelling() that will be used for fetching cached
  spellings from the PTH file.  This doesn't do anything yet.
- Added a hook in Preprocessor::getSpelling() to call PTHLexer::getSpelling()
  when using a PTHLexer.
- Updated PTHLexer to read the offsets of spelling tables in the PTH file.

llvm-svn: 61911