Add CI jobs for more recent versions of flang

[joewallwork]

Closes #3067.

[joewallwork]

@copilot Fix the 'The following signatures couldn't be verified because the public key is not available' errors in the Fortran CI jobs. It looks like we need to import the appropriate public keys.

[Copilot]

@copilot Fix the 'The following signatures couldn't be verified because the public key is not available' errors in the Fortran CI jobs. It looks like we need to import the appropriate public keys.

Fixed in commit 9c71b9a. I updated the Fortran workflow to import the LLVM and Intel repository keys into /etc/apt/trusted.gpg.d, which resolves the missing public key signature verification errors during apt-get update.

[joewallwork]

@copilot Fix pip upgrade in the Fortran Flang CI. This seems to be an issue running on Ubuntu 26.04 because the IFX workflow (which runs on Ubuntu 22.04) seems fine.

[Copilot]

@copilot Fix pip upgrade in the Fortran Flang CI. This seems to be an issue running on Ubuntu 26.04 because the IFX workflow (which runs on Ubuntu 22.04) seems fine.

Fixed in commit b389686. I removed the pip self-upgrade in the flang (Ubuntu 26.04) setup step and now only install lit, which avoids the Debian-managed pip uninstall error.

[joewallwork]

Okay so this lines up with the errors I'd reported on #2968 - it looks like Flang 23 is struggling to access the intrinsic iso_c_binding module.

[joewallwork]

Looking at a local (LLVM 24) build of LLVM/Flang I have lying around, I see that the missing module can be found at

build/lib/clang/24/finclude/flang/x86_64-unknown-linux-gnu/iso_c_binding.mod

[joewallwork]

Looking at a local (LLVM 24) build of LLVM/Flang I have lying around, I see that the missing module can be found at

build/lib/clang/24/finclude/flang/x86_64-unknown-linux-gnu/iso_c_binding.mod

With that Flang, I can compile the following test program fine

program test
  use, intrinsic :: iso_c_binding, only: c_int
  implicit none
end program test