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

[Heejin Ahn via llvm-commits]

aheejin wrote:

Hello. What’s the current status of the main headers? Has any of the C++03 support been removed from the main headers, or we can still use them in the old way by not defining `_LIBCPP_USE_FROZEN_CXX03_HEADERS`?

If it has not been removed, what’s the planned timeline for that?

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