`f18` is no longer available

Hi all,

I just wanted to let everyone that we have just deleted `f18` from LLVM Flang: ⚙ D105811 [flang][driver] Delete `f18` (i.e. the old Flang driver). Many thanks to Michael Kruse and Kiran Chandramohan for reviewing and for their comments!

With this change, the dependency on Clang is no longer optional. Please update your local build scripts accordingly.

Thank you,