Wdeprecated: CollectReachableSymbolsCallback are move constructed/returned by...

Wdeprecated: CollectReachableSymbolsCallback are move constructed/returned by value, so make sure they're copy/moveable

(return by value is in ExprEngine::processPointerEscapedOnBind and any
other call to the scanReachableSymbols function template used there)

Protect the special members in the base class to avoid slicing, and make
derived classes final so these special members don't accidentally become
public on an intermediate base which would open up the possibility of
slicing again.

llvm-svn: 244975