[lldb] Make Broadcaster mutexes non-recursive (#97400)

Non-recursive mutexes encourage better locking discipline and avoid bugs
like #96750, where one can unexpectedly re-enter the critical section on
the same thread, and interrupt a presumed-indivisible operation.

In this case, the only needed fix was to remove locking from some
BroadcastManager functions, which were only called from the Listener
class (and the listener already locked those mutexes to preserve lock
ordering).

While doing that, I noticed we don't have unit tests for these
functions, so I added one.

0 files changed, 0 insertions, 0 deletions