Add LLVM 18 support

[laurynas-biveinis]

• Remove a redundant cast as diagnosed by LLVM 18
• Remove gnu::pure attribute from a method template, as it's being instantiated with void too, where the attribute is illegal. Do not bother specializing the template for the void case.
• Use LLVM 18 for main CircleCI and GitHub Actions runs
• Add LLVM 17 to GitHub Actions old compilers run