Wdeprecated: ByrefHelpers are copy constructed by the ::buildByrefHelpers...

Wdeprecated: ByrefHelpers are copy constructed by the ::buildByrefHelpers helper, make sure they're safely copyable

Make the copy/move ctors protected and defaulted in the base, make the
derived classes final to avoid exposing any slicing-prone APIs.

Also, while I'm here, simplify the use of buildByrefHelpers by taking
the parameter by value instead of non-const ref. None of the callers
care aobut observing the state after the call.

llvm-svn: 244990