Cirrus-CI: Add manual Clang 18 jobs

Work is in progress to import Clang/LLVM 18 as the base system compiler.
llvm18-lite is not yet in the quarterly package set but but will be
available in the not too distant future, and is available for src forks
that switch to latest packages. Add manual llvm18 jobs now so that they
can be used for testing.

Sponsored by: The FreeBSD Foundation