gitlab-ci: add jobs to test with Clang and LLVMFlang on Fedora
We already have `t:debian10-{makefiles,ninja}-clang` jobs covering old
versions of Clang with Makefile and Ninja generators. Complement them
with new `t:fedora37-{makefiles,ninja}-clang` jobs covering recent
versions of Clang.
Fedora 37 now packages LLVMFlang, so enable Fortran tests with that
compiler too.
Issue: #22387