[libc++] Revert OpenBSD-related changes to the documentation

[ldionne]

Summary

This commit partially reverts 0954e2b2d038 and 3fa4cff97480, which
make changes to the libc++ documentation implifying that OpenBSD is
supported. Neither of these changes have been reviewed AFAICT, so
I'm reverting as a matter of enforcing:

1. That changes get reviewed before being committed
2. That we have a discussion and a support plan for supporting OpenBSD officially in libc++

Please note that I would be thrilled to support OpenBSD officially in
libc++, however doing so requires more than adding a note in the docs.
In particular, please make sure you read the note in [1] about setting
up CI testing for OpenBSD.

[1]: https://libcxx.llvm.org/#platform-and-compiler-support

[brad]

Oh I'm sorry I had intended to just fill in the gaps as I perceived it when seeing FreeBSD / NetBSD elsewhere in LLVM and related documentation, but after reading that paragraph which I honestly had not, I see where you're coming from and I would not have added OpenBSD to that section. I would like to see about what is required to have said official designation as libc++ is now our official C++ library for all of our major architectures.

[ldionne]

In D109373#2988355, @brad wrote:

Oh I'm sorry I had intended to just fill in the gaps as I perceived it when seeing FreeBSD / NetBSD elsewhere in LLVM and related documentation, but after reading that paragraph which I honestly had not, I see where you're coming from and I would not have added OpenBSD to that section. I would like to see about what is required to have said official designation as libc++ is now our official C++ library for all of our major architectures.

No worries, that's why I created a review here and CC'd you on it so we can talk. Basically, if you're able to follow the steps here for adding a BuildKite agent on OpenBSD, we'll officially support the platform when all tests are passing (in some cases it might be necessary to XFAIL some tests due to specific reasons, and that's okay).

[ldionne]

Also, as a matter of process, any commit that touches libcxx/ or libcxxabi/ should be reviewed here and get a green checkmark by the libc++/libc++abi review groups before being committed. I'm sure you simply didn't know, so I'm telling you -- it's unfortunate that this can't be enforced automatically with tooling in our current infrastructure, but we might get there eventually!

[brad]

In D109373#2988357, @ldionne wrote:

Basically, if you're able to follow the steps here for adding a BuildKite agent on OpenBSD, we'll officially support the platform when all tests are passing (in some cases it might be necessary to XFAIL some tests due to specific reasons, and that's okay).

I don't have anything to run on at the moment but I will see about trying to set something up.

[brad]

Ok, I'll keep that in mind.

[ldionne]

I'll move forward with this revert, but please do reach out as soon as you have a machine that could run CI on OpenBSD -- I'll help you set up CI bots for it and then we can officially support OpenBSD, that would be great.

I'm really trying to avoid pretending that libc++ supports a platform if it does not actually support it - we've suffered from rotting code due to once-working ports to various platforms in the past and I'd like to stay out of that trap going forward.

libcxx/docs/UsingLibcxx.rst

 On systems where libc++ is provided but is not the default, Clang provides a flag
 called ``-stdlib=`` that can be used to decide which standard library is used.
 Using ``-stdlib=libc++`` will select libc++:
 
 .. code-block:: bash
 
   $ clang++ -stdlib=libc++ test.cpp
 
-On systems where libc++ is the library in use by default such as macOS, FreeBSD
-and OpenBSD, this flag is not required.
+On systems where libc++ is the library in use by default such as macOS and FreeBSD,
+this flag is not required.
 
 
 .. _alternate libcxx:
 
 Using a custom built libc++
 ===========================
 
 Most compilers provide a way to disable the default behavior for finding the

libcxx/docs/index.rst

 
 Libc++ also supports common platforms and architectures:
 
 =============== ========================= ============================
 Target platform Target architecture       Notes
 =============== ========================= ============================
 macOS 10.9+     i386, x86_64, arm64       Building the shared library itself requires targetting macOS 10.11+
 FreeBSD 10+     i386, x86_64, arm
-OpenBSD         i386, x86_64, arm, arm64
 Linux           i386, x86_64, arm, arm64
 Windows         x86_64
 =============== ========================= ============================
 
 Generally speaking, libc++ should work on any platform that provides a fairly complete
 C Standard Library. It is also possible to turn off parts of the library for use on
 systems that provide incomplete support.