[SYCL] Fix bfloat16 to compile with clang < 9.0.0

[aelovikov-intel]

struct A {  // Fails to compile with clang 8.0.1, ok 9.0.0
    constexpr A& operator=(const A&) = default;
    ~A() = default;
};
struct B {  // Ok with either version
    ~B() = default;
    constexpr B& operator=(const B&) = default;
};

int main() {
    A a;
    B b;
    return 0;
}

Since the workaround isn't hurting readability/maintenance might just do it even if the support for such old clang is questionable.

[aelovikov-intel]

https://godbolt.org/z/zj14hTKPe for the case from the PR's description.