[flang] Fine-tune function result equivalence checking (#70260)

When a separate module function's definition has a redundant interface
-- it's defined with MODULE FUNCTION, not MODULE PROCEDURE -- the check
for result type equivalence needs to allow for character lengths that
are the results of specification expressions. At present,
identical-looking length specification expression don't compare equal,
since they can refer to distinct dummy argument symbols. Ensure just
that they are both constant or not, and if constant, that the lengths
have the same value.