Avoid narrowing warnings in __bitset constructor

When <bitset> is compiled with warnings enabled, on a platform where
size_t is 4 bytes, it results in errors similar to:

    bitset:265:16: error: non-constant-expression cannot be narrowed
    from type 'unsigned long long' to '__storage_type' (aka 'unsigned
    int') in initializer list [-Wc++11-narrowing]
        : __first_{__v, __v >> __bits_per_word}
                   ^~~
    bitset:676:52: note: in instantiation of member function
    'std::__1::__bitset<2, 53>::__bitset' requested here
            bitset(unsigned long long __v) _NOEXCEPT : base(__v) {}
                                                       ^

Fix these by casting the initializer list elements to __storage_type.

Reviewers: mclow.lists, EricWF
Differential Revision: https://reviews.llvm.org/D23960

llvm-svn: 280543