Work around MSVC bug in IntrusiveRefCntPtr.h
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@202988 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,GPUOpen-Drivers/llvm,chubbymaggie/asap,llvm-mirror/llvm,dslab-epfl/asap,dslab-epfl/asap,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,chubbymaggie/asap,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,chubbymaggie/asap,dslab-epfl/asap,chubbymaggie/asap,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,chubbymaggie/asap,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,chubbymaggie/asap,dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm