Big change in the way ObjectFile file contents are managed. We now

mmap() the entire object file contents into memory with MAP_PRIVATE.
We do this because object file contents can change on us and currently
this helps alleviate this situation. It also make the code for accessing
object file data much easier to manage and we don't end up opening the
file, reading some data and closing the file over and over.

llvm-svn: 148017