[clang] [llvm] [LLVMABI] Handle Atomic types in the LLVM ABI library (PR #221367)

[Nikita Popov via cfe-commits]

nikic wrote:

Uh, are we sure that what Clang is doing here is actually correct?

At least the ABI used by Clang is not the same as the one used by GCC: https://gcc.godbolt.org/z/Kec3Eqcz6 GCC seems to treat the struct of `_Atomic(float)` the same as the struct of `float`.

The x86-64 SysV ABI specification unfortunately has no hits for `_Atomic`.

cc @efriedma-quic 

https://github.com/llvm/llvm-project/pull/221367