Addendum to r254141: The call to vm_radix_insert() in vm_page_cache() can
reclaim the last preexisting cached page in the object, resulting in a call
to vdrop().  Detect this scenario so that the vnode's hold count is
correctly maintained.  Otherwise, we panic.

Reported by:	scottl
Tested by:	pho
Discussed with:	attilio, jeff, kib
Import lldb as of SVN r188801

(A number of files not required for the FreeBSD build have been removed.)

Sponsored by:	DARPA, AFRL
There is no need to hold the freelist lock around alloc/free of
software descriptors.  This also silences WITNESS warnings when
the software descriptors are allocated with M_WAITOK.

MFC after:	1 week
Merge lldb r188801 to contrib/llvm/tools/lldb/