Various improvements to MemoryBuffer::getFile:

- Use a RAII object to close the FD.
- Use sys::StrError instead of thread-unsafe strerror calls.
- Recover gracefully if read returns zero. This works around an issue on
  DragonFlyBSD where /dev/null has an st_size of 136 but we can't read 136 bytes
  from it.

llvm-svn: 100106