[lldb] Simplify creating RegisterSets in DynamicRegisterInfo (#208592)
`DynamicRegisterInfo::GetRegisterSetIndexByName` is only used in
`DynamicRegisterInfo::SetRegisterInfo` so I inlined it and removed the
function. I also noticed that `m_set_names` is only used in this
function for setup, so I made it a local variable instead of a member.