[lldb/Reproducers] Change the way we instrument void* arguments

The reproducer instrumentation cannot automatically serialize and
deserialize void* arguments. Currently we deal with this by explicitly
preventing these methods from being instrumented. This has the undesired
side effect of breaking replay when that method returns a value later
used by another SB API call.

The solution is to change our approach and instrument these methods.
Instead of using the DUMMY macro, we just make (de)serialization of the
void pointer a NOOP and always return a nullptr.