add llvm 18 support

[tchaikov]

since llvm 18 is around the corner, can we add support for it? i will create a pull request for this if this is acceptable. it would be great if we can even support llvm 19. as, under some circumstances, we need to test the compatibility of a library to see if it is compatible with the latest clang. i would be happy to create a pull request for this as well.

[aminya]

We should wait for the release
https://github.com/llvm/llvm-project/releases

Once released 18 should be added to this for the URL-based installers

setup-cpp/src/llvm/llvm_url.ts

Line 14 in 2908244

export const VERSIONS: Set<string> = getVersions([

And the default version should be updated here:
https://github.com/aminya/setup-cpp/blob/master/src/versions/default_versions.ts

[aminya]

@tchaikov LLVM18 is released with binaries. would you want to make a PR?