[libcxx-commits] [libcxx] Reapply "[libc++][C++03] Copy the LLVM 19 headers (#108999)" (PR #112127)

[A. Jiang via libcxx-commits]

frederick-vs-ja wrote:

This PR caused CI failures from `headers_in_modulemap.sh.py`. Error message indicates:
```
Header __cxx03/[...] seems to be missing from the modulemap!
```

Can we just exclude the `__cxx03` subdirectory from `is_in_modulemap`?

https://github.com/llvm/llvm-project/pull/112127