[lldb] Remote disk file/directory completion for platform commands
- Extended the gdb-remote communication related classes with disk file/directory completion functions;
- Added two common completion functions RemoteDiskFiles and RemoteDiskDirectories based on the functions above;
- 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>.
- Added related tests for client and server;
- Updated docs/lldb-platform-packets.txt.
Reviewed By: labath
Differential Revision: https://reviews.llvm.org/D85284