__threading_support: fix windows build

The build was broken as there was no overload for long and
std::chrono::nanoseconds.  Add an explicit conversion to use the
operator+.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294698 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed