When loading kernel binary, use DownloadObjectAndSymbolFile last

When lldb starts a kernel debug session, it has the UUID of the
kernel binary.  lldb will try three different methods to find a
binary and symbol file for this UUID.  Currently it calls out to
Symbols::DownloadObjectAndSymbolFile() first, which may be the
slowest method when a DBGShellCommand can find the UUID on a
network filesystem or downloaded from a server.

This patch tries the local searches first, then falls back to that
method.

Differential Revision: https://reviews.llvm.org/D157165