Port `APInt` from LLVM

* [`APInt.h`](https://github.com/llvm/llvm-project/blob/bbc6504b3d2f237ed7e84dcaecb228bf2124f72e/llvm/include/llvm/ADT/APInt.h)
* [`APInt.cpp`](https://github.com/llvm/llvm-project/blob/bbc6504b3d2f237ed7e84dcaecb228bf2124f72e/llvm/lib/Support/APInt.cpp)
* [`APIntTest.cpp`](https://github.com/llvm/llvm-project/blob/bbc6504b3d2f237ed7e84dcaecb228bf2124f72e/llvm/unittests/ADT/APIntTest.cpp)