[lldb][NFCI] Deprecate SBValue::GetOpaqueType

This method, as far as I can ascertain, is non-trivial to actually use
to work with (if not impossible). It doesn't make sense to use from
Python and you do not have access to the accompanying TypeSystem, so it
doesn't really do anything useful.

A possible follow-up is to gut the implementation and have it return `nullptr`.

Differential Revision: https://reviews.llvm.org/D153918