Fix std::has_unique_object_representations for _BitInt types with padding bits

"std::has_unique_object_representations<_BitInt(N)>" was always true,
even if the type has padding bits (since the trait assumes all integer
types have no padding bits). The standard has an explicit note that
this should not hold for types with padding bits.

Differential Revision: https://reviews.llvm.org/D125802