[Request] Add support of Clang 18

[Hourier]

As you can see below, LLVM v18 was released:
https://github.com/llvm/llvm-project/releases

So, I would be appreciated if this is supported.

[dragondive]

Not sure if this repository is still being actively maintained, but I created this PR: #21

It turns out Clang versions up to 18 and Ubuntu release 24.04 LTS (Noble Numbat) can be used with this action already. My PR only tests this and makes it more explicit.