[llvm-branch-commits] [libcxx] [llvm] [libc++][C++03] Use `__cxx03/` headers in C++03 mode (PR #109002)

[Louis Dionne via llvm-branch-commits]

https://github.com/ldionne commented:

Let's write down the order for landing this in smaller pieces:

1. Land the `#if 0` change which addresses the formatting issues.
2. Land the CMake changes that start installing the C++03 headers.
3. Switch to `#if __cplusplus < C++11 && USE_FROZEN_HEADERS` (or whatever), and also add the CI with XFAILs. If there are too many XFAILs, the CI bit should probably be left to a separate patch.

We can treat even C++23-only headers the same for now, by including `__cxx03/__config` in them. That's the status quo and we should start with that. However, as a follow-up, we should go through all of the >= 11 headers and make them "not care" about the C++03 frozen headers. We could in addition perhaps diagnose the use of `-std=c++03` in the normal `__config` file with an error.

https://github.com/llvm/llvm-project/pull/109002