[flang-dev] `f18` is no longer available

[Andrzej Warzynski via flang-dev]

Hi all,

I just wanted to let everyone that we have just deleted `f18` from LLVM 
Flang: https://reviews.llvm.org/D105811. 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,
-Andrzej