Number of bits needed to represent the largest enumerator of bitmask enum E.
Declared in <llvm/ADT/BitmaskEnum.h>
template<typename E>
requires is_bitmask_enum<E>::value
constexpr unsigned int BitWidth = llvm::bit_width_constexpr(
uint64_t{llvm::to_underlying(E::LLVM_BITMASK_LARGEST_ENUMERATOR)});