[Reproducers] Add LLDB_RECORD_DUMMY

Add a macro that doesn't actually record anything but still toggles the
API boundary. Removing just the register macros for lldb::thread_t
wasn't sufficient on NetBSD because the serialization logic needed the
underlying type to be complete.

This macro should be used by functions that are currently unsupported,
as they might trip the API boundary logic. This should be easy using the
lldb-instr tool.

llvm-svn: 355709