[ADT] Try to work around a crash in MSVC.

[ADT] Try to work around a crash in MSVC.

Putting `sizeof(T) <= 16` into the parameter of a `std::conditional`
causes every version of MSVC I've tried to crash:

  https://godbolt.org/g/eqVULL

Really frustrating, but an extra layer of indirection through an
instantiated type gives a working way to access this computed constant.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336170 91177308-0d34-0410-b5e6-96231b3b80d8