[lldb] Remote disk file/directory completion for platform commands

1. Extended the gdb-remote communication related classes with disk file/directory
   completion functions;
2. Added two common completion functions RemoteDiskFiles and
   RemoteDiskDirectories based on the functions above;
3. Added completion for these commands:
   A. platform get-file <remote-file> <local-file>;
   B. platform put-file <local-file> <remote-file>;
   C. platform get-size <remote-file>;
   D. platform settings -w <remote-dir>;
   E. platform open file <remote-file>.
4. Added related tests for client and server;
5. Updated docs/lldb-platform-packets.txt.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D85284