[lldb] [client] Support for multiprocess extension

Add a minimal support for the multiprocess extension in gdb-remote
client.  It accepts PIDs as part of thread-ids, and rejects PIDs that
do not match the current inferior.

Differential Revision: https://reviews.llvm.org/D99603

GitOrigin-RevId: b601c6719226fb83c43dae62a581e5ee08bfb169
5 files changed