Add --lldb-command command-line option to lldb-gdbserver

lldb-gdbserver now takes zero or more --lldb-command {command} options
that get executed by the command interpreter prior to starting up
a child debuggee process.  The short form is -c {command}.  This is
similar to how lldb-platform works.

This command can be used to enable logging in more than just the
gdb-remote log channel.  Here is an example startup sequence that allows
logging of lldb host and process log categories to one file and
gdb-remote packets to another file:

lldb-gdbserver -c 'log enable -f lgs.log lldb process host' \
  -c 'log enable -f lgs_packets.log gdb-remote packets' \
  localhost:5432 ./some_exe

llvm-svn: 199761