[lldb] Use llvm::Optional for port in LaunchGDBServer

Previously we used UINT16_MAX to mean no port/no specifc
port. This leads to confusion because 65535 is a valid
port number.

Instead use an optional. If you want a specific port call
LaunchGDBServer as normal, otherwise pass an empty optional
and it will be set to the port that gets chosen.
(or left empty in the case where we fail to find a port)

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D92035