[libc++][modules] Add using_if_exists attribute (#77559)

[gwllx]

libc on macOS does not provide at_quick_exit or quick_exit. This allows modules to build on macOS and defer any errors to usage site of these symbols.

Fixes: #77559

[github-actions]

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be
notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write
permissions for the repository. In which case you can instead tag reviewers by
name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review
by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate
is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Gareth Williamson (gwllx)

Changes

libc on macOS does not provide at_quick_exit or quick_exit. This allows modules to build on macOS and defer any errors to usage site of these symbols.

CC: @mordante

---

Full diff: https://github.com/llvm/llvm-project/pull/78909.diff

1 Files Affected:

• (modified) libcxx/modules/std.compat/cstdlib.inc (+2-2)

diff --git a/libcxx/modules/std.compat/cstdlib.inc b/libcxx/modules/std.compat/cstdlib.inc
index 9333d848707103..a45a0a1caf8ba9 100644
--- a/libcxx/modules/std.compat/cstdlib.inc
+++ b/libcxx/modules/std.compat/cstdlib.inc
@@ -16,10 +16,10 @@ export {
   // [support.start.term], start and termination
   using ::_Exit;
   using ::abort;
-  using ::at_quick_exit;
+  using ::at_quick_exit _LIBCPP_USING_IF_EXISTS;
   using ::atexit;
   using ::exit;
-  using ::quick_exit;
+  using ::quick_exit _LIBCPP_USING_IF_EXISTS;
 
   using ::getenv;
   using ::system;

[mordante]

Thanks a lot for testing and supplying this patch! I expect the CI passes once it does I'll land the patch.