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

[Louis Dionne via llvm-commits]

================
@@ -1047,12 +1047,15 @@ set(files
   wctype.h
   )
 
+add_subdirectory(__cxx03)
----------------
ldionne wrote:

I would suggest handling the C++03 headers by simply adding them to the list of files above instead. That way we don't need any additional logic, no new targets, etc. If we're worried that this file is going to become very long (and mention a lot of files that we'd rather forget), we can do something like

```cmake
set(cxx03_headers
  ... long list ...
)
foreach (f ${cxx03_headers})
  list(APPEND files ${f})
endforeach()
```

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