[flang] Reduce implicit interface compatibility warnings due to length (#69385)

When a procedure with an implicit interface is called from two call
sites with distinct argument types, we emit a warning. This can lead to
a lot of output when the actual argument types are differing-length
character, and the warnings are less important since the procedure may
well have been defined with assumed-length character dummy arguments. So
let cases like CALL MYERROR('ab'); CALL MYERROR('abc') slide by without
a warning.