NPL: Fix single step workaround

While refactoring the code in r293046 I made a very basic error -
relying on destructor side-effects of a copyable object. Fix that and
make the object non-copyable.

This fixes the tests on the platforms that need this workaround, but
unfortunately we don't have a way to make a more platform-agnostic test
right now.

llvm-svn: 295345