Add LFortran to CI workflow

[joewallwork]

Might as well include LFortran as well as Flang and IFX. Preliminary local testing suggests that it supports assumed shape (like IFX, unlike Flang) but fails in some of the cases IFX also fails for.

[vchuravy]

Personally, I would prefer focusing on upstream flang. We can add lfortran/ifx to CI but under the clear warning that this is best effort only and we may drop them at anytime.

[joewallwork]

Fair enough. I'll leave this as low priority.