tree dff6d7f9658e37fc66d1711fb479bcc0019addd2
parent 3a644127ceca1554e653758eaaa967b4e220d06b
author Nikolas Klauser <nikolasklauser@berlin.de> 1734782508 +0100
committer Copybara-Service <copybara-worker@google.com> 1734782818 -0800

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

This patch implements the forwarding to frozen C++03 headers as
discussed in
https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc. In the
RFC, we initially proposed selecting the right headers from the Clang
driver, however consensus seemed to steer towards handling this in the
library itself. This patch implements that direction.

At a high level, the changes basically amount to making each public
header look like this:

```
// inside <vector>
#ifdef _LIBCPP_CXX03_LANG
#  include <__cxx03/vector>
#else
  // normal <vector> content
#endif
```

In most cases, public headers are simple umbrella headers so there isn't
much code in the #else branch. In other cases, the #else branch contains
the actual implementation of the header.

GitOrigin-RevId: b9a2658a3e8bd13b0f9e7a8a440832a95b377216