[flang] Check for another case of ambiguous generic resolution

When specific procedures of a generic have dummy procedures,
underspecified actual procedures can match more than one specific
procedure.  This can happen with actual procedures that are
externals with implicit interfaces, including the completely
unspecified case of a PROCEDURE() or EXTERNAL that doesn't even
differentiate between a subroutine and a function.

Generic resolution can already handle cases of ambiguous resolution
due to the use of NULL() actual arguments with no MOLD= arguments
to define their types.  Extend the handling of ambiguous actual
arguments to include the case of underspecified actual procedures.

Differential Revision: https://reviews.llvm.org/D140151