Change the DarwinKernel DyanmicLoader to maintain a persist list

of kernel extensions (kexts) that have been loaded into the kernel.
Now when we hit the "kexts have changed" breakpoint we can avoid
adding kexts multiple times, and can properly detect kext unloads
and remove them from the Target's list of modules.

<rdar://problem/13107639>
<rdar://problem/13191016>

llvm-svn: 175489