[lldb] Code cleanup: Simplify SBCommandReturnObject

A simplification for D67589. m_opaque_up can never be nullptr (unless one calls
a ctor with nullptr or one uses SetLLDBObjectPtr with nullptr).

Also protected SetLLDBObjectPtr is not used anywhere (I haven't found it would
ever be used).

Differential revision: https://reviews.llvm.org/D68083

llvm-svn: 372976