Added clang-18 to default settings.yml

[mkmkme]

Changelog: Feature: Added support for Clang 18.
Docs: conan-io/docs#3637

Clang-18 is being downloaded by default from apt.llvm.org, and it gets stable tomorrow. I think it's time to add it to the default settings.yml :)

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[memsharded]

Thanks for your contribution @mkmkme

It would be good to have explicit confirmation that clang 18 will not change anything, like the cppstd current default of gnu17

[mkmkme]

So far they have not published the release notes for clang itself, only for libc++ 18, and at least this does not change anything: https://libcxx.llvm.org/ReleaseNotes/18.html

As for standard, my clang-18.1.0 taken from apt.llvm.org states that default standard for C is gnu17 except for PS4 where it's gnu99 (i.e. not changed). Default one for C++ is gnu++17 (I also believe it's not changed).

If there's anything else worth checking, please tell me and I'll check at least on my installation. But I hope official release notes will be also released quite soon.

[memsharded]

Merged! It will be in next 2.2. Many thanks!
In the meantime, recall that it is possible to add it to your settings_user.yml