[lldb] Change definition of DisassemblerCreateInstance

DissassemblerCreateInstance is a function pointer whos return type is
`Disassembler *`. But Disassembler::FindPlugin always returns a
DisassemblerSP, so there's no reason why we can't just create a
DisassemblerSP in the first place.

Differential Revision: https://reviews.llvm.org/D150235