Don't load non-kexts in darwin kernel debug; handle unslid segs

We have some non-kexts in the binary list in the Darwin kernel
in some situations.  The binary has likely already been loaded;
check if it has been, and don't re-load it.  Also, if we do need
to load it at this point, if in-memory segment vmaddrs have not
been updated to the actual load addresses, calculate a fixed slide
for the in-memory image and apply that slide to the ondisk binary.

Differential Revision: https://reviews.llvm.org/D145547
rdar://106343477