Work around MSVC bug in IntrusiveRefCntPtr.h

The build was failing with:

  error C2664: 'std::atomic_int::atomic_int(const std::atomic_int &)' : cannot convert argument 1 from 'int' to 'const std::atomic_int &'

Apparently "std::atomic_int x(0)" doesn't work, but "std::atomic<int> x(0)"
does.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202988 91177308-0d34-0410-b5e6-96231b3b80d8