Clang C++17 Feature: Core bits for P0154R1 - Hardware interference size

[h-vetinari]

id: P0154R1
paper: https://wg21.link/P0154R1

Though this is primarily a library feature, there are some macros necessary that currently only work with GCC, and not Clang. There was an appeal before LLVM 15 for Clang to match GCC, but it landed as GCC-only.

I guess this got forgotten in the meantime, so raising an issue to track. More specifically, https://libcxx.llvm.org/Status/Cxx17.html says:

P0154: This is currently not implemented for clang because clang does not implement the required macros.

[llvmbot]

@llvm/issue-subscribers-c-17

[h-vetinari]

Just noticed that this duplicates #60174...