Workaround bot failure with explicit conversion to MDTuple*

A bot is failing [1] after r234326, apparently because this code doesn't
do what I think it should:

    template <class U>
    explicit MDTupleTypedArrayWrapper(
        const U &Tuple,
        typename std::enable_if<
            std::is_constructible<const MDTuple *, U>::value>::type * = nullptr)
        : N(Tuple) {}

Just be explicit for now.

[1]: http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-incremental_build/8201/

llvm-svn: 234329