bump to clang 18

[h-vetinari]

Clang has a 6 month release cadence, and the last few years we've updated to an even release in the fall (#4991, #3261, #2010). We bumped to clang 17 relatively recently (mainly due to waiting for the stdlib rollout, and then because GCC 13.3 got stuck), but IMO we can keep the previous rhythm here.

In particular, development on LLVM 17 has stopped completely. The same is true for LLVM 18 actually, but at least it contains 6 months more of features & fixes. Upstream is currently doing point-releases for 19.x, which will stop with the release of v20 early next year. We can then do a bump to GCC 14 & Clang 19 in the spring again (after the release of GCC 15 resp. the corresponding 14.x point-release).

To-Do list based on docs

• Update all respective feedstocks (listing only those for activation; they wouldn't pass if the implementation wasn't complete)
  • LLVM 18.1.8 clang-compiler-activation-feedstock#131 (osx)
  • Add clang 18 ctng-compiler-activation-feedstock#104 (linux; not part of global pinning)
  • conda-forge/clang-win-activation-feedstock@97b64c0 (win; not part of the global pinning)
  • bump to 18.1.8 r_clang_activation-feedstock#7 (clang for r-feedstocks; not part of global pinning)
• Tested that compilers work
  • WIP: Test new compilers scipy-feedstock#250
• Checked release notes for ABI-relevant changes
  • Clang potentially breaking changes
    • Minor mangling change for function templates
  • Clang ABI changes
    • changed behaviour of how __int128 arguments are passed (this was a long-running effort by the Rust folks AFAIU)
    • Minor changes (IMO) of the kind we had ignored in 17, 16, 15 & 14

Previously: #4890 #4991 #4215 #3731 #3290 #3261 #2802 #2010

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

[h-vetinari]

This was discussed in the core call and received no opposition. In it goes 🙃