[flang] Address regression (calls to assumed-length character function dummy procedures)

A recent fix beefed up semantics checking to catch the case of a call
to an external assumed-length character function; this check has false
positives in the case of an assumed-length character function that is
a dummy procedure.  These do have a length that is passed in extra
compiler-created arguments.  This patch refines the check and undoes some
changes to tests.

Differential Revision: https://reviews.llvm.org/D126390