[clang] [llvm] [CMake] Add support for building on illumos (PR #74930)

[Rainer Orth via llvm-commits]

rorth wrote:

I wonder what the current flurry of patdhes to support Illumos is about.  I'd originally filed Issue #53919 to make clear what's needed from the Illumos project to keep it as an LLVM target, given that they haven't contributed either bug reports, code or testing in a long time.  Without that, keeping Illumos somehow in a gray zone between supported and unsupported, this doubles my workload for Solaris patches:

- Whenever I add or remove a feature, I need to investigate what needs to be done on the Illumos side.
- Depending on the outcome, I need to add separate Illumos support code whenever it and Solaris differ.
- And ultimately, I need to test all my patches not only on Solaris, but also no Illumos.

This could only be avoided if the Illumos guys (or someone else) reliably perform those tasks instead.  However, I don't see any contributions from them, so the situation hasn't changed since the Issue was filed.

It also means that cleanup that could be done on the Solaris side (I'd like to rip out [[sanitizer_common] Support Solaris < 11.4 in GetStaticTlsBoundary](https://reviews.llvm.org/D120059) as soon as Solaris 11.3 support has been ripped out from GCC; 11.4 doesn't need it, but Illumos would AFAIK).

I'm perfectly happy to work with them to develop code that works on both variants (Illumos and Solaris), but I'm opposed to have untested/barely tested code in that creates massive work for me.

https://github.com/llvm/llvm-project/pull/74930