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

[Xavier Roche via cfe-commits]

xroche wrote:

> Thank you for working on this! One more thing to be sure to test are constant expressions and constant initialization support. e.g.,

C23 rejects constexpr atomic objects unless I'm mistaken ? :thinking: 

> 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).

:+1: 

I added tests for that, _BitInt(3) overflow, initialization, operators, and Clang’s absent header names.


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