The bitset(unsigned long long) constructor was broken by the constexpr additions only on 32 bit platforms.  Fixed.  This addresses http://llvm.org/bugs/show_bug.cgi?id=15444.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@176559 91177308-0d34-0410-b5e6-96231b3b80d8