Sanity check --max-gdbserver-port

In mail
	[lldb-dev] Remote debugging a docker process
	https://lists.llvm.org/pipermail/lldb-dev/2019-March/014795.html
user was confused by --min-gdbserver-port and --max-gdbserver-port options
being ignored. I think there is even a bug that --max-gdbserver-port is upper
exclusive limit (and not upper inclusive limit appropriate for max).

At least this patch should catch such mistake by an error message. The question
is whether --max-gdbserver-port should not be changed to really be max and not
max+1 but that would break backward compatibility.

Now the mail example does produce:
	error: --min-gdbserver-port (5001) is not lower than --max-gdbserver-port (5001)

Differential Revision: https://reviews.llvm.org/D58962

llvm-svn: 355554