Factor out hardcoding of the string "clang<version". When doing the
work to identify stale versions of clang tree-wide, these instances
cause extra work.

Refactor this usage into LLVM_VER.

Note: this refactoring causes portlint to complain but IMHO this
annoyance is worth tolerating to help ease maintainability.

PR: 239737, 239738
Approved by: maintainer