Flang is now enabled for pre-merge checks

Hi!

Flang project is now enabled for pre-merge checks both for Linux and Windows. Big thanks to Mehdi Chinoune for following it through and making sure that tests and builds are working well on Windows.

See also https://github.com/google/llvm-premerge-checks/issues/287 .

Kind regards,
Mikhail

This is great news, many thanks for all your efforts!

-Andrzej