[clang] [compiler-rt] [libcxx] [Clang] Support atomic operations on _BitInt(N) (PR #204815)

[Aaron Ballman via llvm-commits]

https://github.com/AaronBallman commented:

Thank you for working on this! One more thing to be sure to test are constant expressions and constant initialization support. e.g.,
```
constexpr _Atomic _BitInt(12) i = 100; // okay
constexpr _Atomic _BitInt(3) j = 100; // error due to overflow
i++;
--i; 
// etc
```
We should also make sure there is *not* an `ATOMIC_BITINT_LOCK_FREE` macro being defined nor an `atomic_bit_int` type name (one isn't permitted by the standard, which is different from the other builtin types).

https://github.com/llvm/llvm-project/pull/204815