Force libcompression calls to be enabled when building on Darwin

systems.  It has been available in the OS over over three years
now.  If lldb doesn't link against -lcompression, it should be an
error.

Allocate a scratch buffer for libcompression to use when decoding
packets, instead of it having to allocate & free one on every call.

Fix a typeo with the size of the buffer that compression_decode_buffer()
is expanding into.

<rdar://problem/41601084> 

llvm-svn: 349563