[libcxx-commits] [libcxx] [libc++] Inline __has_feature and __has_extension uses (PR #133634)

[Louis Dionne via libcxx-commits]

https://github.com/ldionne approved this pull request.

I'm fine with this patch (after rebasing and applying my nit comment). Reasoning:
- this patch doesn't actually change the ability of users to (incorrectly) override things like `_LIBCPP_HAS_ASAN`, since we unconditionally set its value based on `__has_feature(address_sanitizer)` before this patch. Thus, I don't think this can break anyone who isn't already broken.
- We already have a notion of whether the library has been built with ASAN enabled in the `__config_site`, and we use that to guard whether container annotations are enabled in `basic_string`, which is where this matters since we externally instantiate `string` in the dylib.

Thus, I think this change is fine and is just a cleanup.

https://github.com/llvm/llvm-project/pull/133634