[lldb] [llgs] Include process ID in stop responses

Include the process identifier in the `T` stop responses when
multiprocess extension is enabled (i.e. prepend it to the thread
identifier).  Use the exposed identifier to simplify the fork-and-follow
tests.

The LLDB client accounts for the possible PID since the multiprocess
extension support was added in b601c671.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D127192