bitmask_enum.h is essentially a copy of llvm/ADT/BitmaskEnum.h, with some minor
cleanup and renaming.
The bit_ceil function is a placeholder for std::bit_ceil, which we can use once
compiler-rt can use c++20.
These utilities will be used to simplify bitfield enum usage in upcoming
ORC-RT patches.
This is a substitute for std::apply, which we can't use until we move to c++17.
apply_tuple will be used in upcoming the upcoming wrapper-function utils code.
