Add a new PlatformDarwinKernel for kernel debugging. This Platform

plugin will index the kext bundles on the local filesystem when
created.  During a kernel debug session, when the DynamicLoader
plugin needs to locate a kext by name like
"com.apple.com.apple.filesystems.autofs", the Platform can quickly
look for a UUID match in those kernel debug kit directories it
previously indexed.

I'm still working on profiling the performance impact of the inital
kext bundle scan; there will likely need to be a switch to enable
or disable this plugin's scan.

This only affects Mac kernel debugging and the code is only built
on Apple systems because of some use of low-level CoreFoundation
to parse plists.

<rdar://problem/13503583> 

llvm-svn: 178827