[flang] CHARACTER(*) return does not require explicit interface

Fortran 2018 15.4.2.2(4)(c) says nonassumed or explicit non-constant
length parameter require explicit interface. The "nonassumed" part was
missing in f18 characteristic analysis causing CanBeCalledViaImplicitInterface
to return false for `CHARACTER(*) function foo()` like interfaces.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D88075