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

[jeanPerier]

Summary

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.

flang/lib/Evaluate/characteristics.cpp

if (typeAndShape->Rank() > 0) {
       return false; // 15.4.2.2(4)(a)
     } else {
       const DynamicType &type{typeAndShape->type()};
       switch (type.category()) {
       case TypeCategory::Character:
         if (const auto *param{type.charLength()}) {
           if (const auto &expr{param->GetExplicit()}) {
             return IsConstantExpr(*expr); // 15.4.2.2(4)(c)
+          } else if (param->isAssumed()) {
+            return true;
           }
         }
         return false;
       case TypeCategory::Derived:
         if (!type.IsPolymorphic()) {
           const auto &spec{type.GetDerivedTypeSpec()};
           for (const auto &pair : spec.parameters()) {
             if (const auto &expr{pair.second.GetExplicit()}) {