PTH emission:

- Output 32 bit integers using bit-shifting + write of individual bytes.
  This is motivated because we aren't guaranteed to load 32-bit ints of the mmaped PTH file at 4-byte offsets.
- Don't emit flags for IdentifierInfos.  These are lazily populated by the Preprocessor/Parser.
- Only write out tokens for files with absolute paths.  This is potentially temporary, but simplifies things for now.

llvm-svn: 60435