Port `APSInt` from LLVM

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