[flang] Emit error about missing interface when needed

[klausler]

Summary

When a procedure pointer with no interface is called by a
function reference, complain about the lack.

[PeteSteinfeld]

All builds, tests, and looks goodl.

flang/lib/Semantics/expression.cpp

if (chars->functionResult) {
         return Expr<SomeType>{
             ProcedureRef{std::move(proc), std::move(arguments)}};
       } else {
         // Not a procedure pointer, so type and shape are known.
         return TypedWrapper<FunctionRef, ProcedureRef>(
             DEREF(result.GetTypeAndShape()).type(),
             ProcedureRef{std::move(proc), std::move(arguments)});
       }
+    } else {
+      Say("Function result characteristics are not known"_err_en_US);
     }
   }
   return std::nullopt;
 }
 
 MaybeExpr ExpressionAnalyzer::MakeFunctionRef(
     parser::CharBlock intrinsic, ActualArguments &&arguments) {
   if (std::optional<SpecificCall> specificCall{

flang/test/Semantics/resolve09.f90

 !ERROR: Cannot call subroutine 'x' like a function
 z = x()
 end
 
 subroutine s
   !ERROR: Cannot call function 'f' like a subroutine
   call f
   !ERROR: Cannot call subroutine 's' like a function
+  !ERROR: Function result characteristics are not known
   i = s()
 contains
   function f()
   end
 end
 
 subroutine s2
   ! subroutine vs. function is determined by use