[lldb] [gdb-remote server] Refactor handling qSupported

Refactor handling qSupported to use a virtual HandleFeatures() method.
The client-provided features are split into an array and passed
to the method.  The method returns an array of server features that are
concatenated into the qSupported response to the server.

The base implementation of HandleFeatures()
in GDBRemoteCommunicationServerCommon now includes only flags common
to both platform server and llgs, while llgs-specific flags are inserted
in GDBRemoteCommunicationServerLLGS.

Differential Revision: https://reviews.llvm.org/D100140