Move ownership of Pass File's to the MachoLinkingContext.

Move ownership of Pass File's to the MachoLinkingContext.

In trying to fix the leaks in the MachO lld codebase, we need to have
a better model for file and atom ownership.  Having the context own
everything seems like the simplest model, so change all the passes to
allocate File's on the context instead of owning files as a member.

llvm-svn: 264004