Implement __builtin_clear_padding (used by libstdc++ for P0528R3/P1123R0)

[comex]

This is basically identical to #46030 ("Implement __builtin_zero_non_value_bits which was recently added to MSVC") except for GCC instead of MSVC.

C++20 includes P0528R3 and P1123R0, which together require std::atomic<T>::compare_exchange_strong and std::atomic_ref<T>::compare_exchange_strong to ignore any padding bits that exist in T. libstdc++ implements this using a builtin added to GCC, __builtin_clear_padding. Clang should implement this so that the feature works correctly when using libstdc++ with Clang.

See also:

• C++20's P0528R3/P1123R0 not implemented (atomic ignoring padding bits) #64829 for libc++ support
• GCC bug 111077 (libstdc++'s implementation is not correct in one case)

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[ghostway0]

When is alignment decided for specific fields? Isn't it in LLVM itself, later in the pipeline? If I recalled correctly, this might need to be an intrinsic - right?