Change the indexing done for kernel/kext directories to be recursive.

Change the indexing done for kernel/kext directories to be recursive.
Also re-write how most of the directory indexing is done - as it has
grown over the years, it has become a bit of a mess and was overdue
for a cleanup.

Most importantly, this allows you to specify a directory with the
platform.plugin.darwin-kernel.kext-directories setting and now lldb
will search for kexts and kernels in those directories recursively.

<rdar://problem/20754467>

llvm-svn: 277789