LWG-3561 Issue with internal counter in discard_block_engine

Discussion from the meta-issue #2236:

• @MattStephanson

  LWG-3561: This would involving changing discard_block's internal _Nx counter and template arguments from int to size_t, which I think would be an ABI break. I think the most we could do now is add a // TRANSITION, ABI comment, and maybe a static_assert for discard_block_engine that its parameters must be <= INT_MAX.

• @CaseyCarter

  I think we could greatly improve discard_block_engine by divorcing it completely from the TR1-era discard_block. We could even make it tolerate p > INT_MAX, but I agree that r > INT_MAX is not supportable without breaking ABI.

• @timsong-cpp

  Couldn't you change the type of _Nx only if r > INT_MAX?
  Edit: looks like MSVC doesn't check narrowing from size_t to int in the NTTP, hmm...is it an ABI break if the existing thing compiles but totally doesn't work?