[flang] Add semantic check for named constant as function result

[peixin]

Summary

Similar to procedure argument, the function result cannot be one
named constant.

flang/lib/Semantics/check-declarations.cpp

if (IsNamedConstant(symbol)) {
           "A dummy argument may not also be a named constant"_err_en_US);
     }
     if (!symbol.test(Symbol::Flag::InDataStmt) /*caught elsewhere*/ &&
         IsSaved(symbol)) {
       messages_.Say(
           "A dummy argument may not have the SAVE attribute"_err_en_US);
     }
   } else if (IsFunctionResult(symbol)) {
+    if (IsNamedConstant(symbol)) {
+      messages_.Say(
+          "A function result may not also be a named constant"_err_en_US);
+    }
     if (!symbol.test(Symbol::Flag::InDataStmt) /*caught elsewhere*/ &&
         IsSaved(symbol)) {
       messages_.Say(
           "A function result may not have the SAVE attribute"_err_en_US);
     }
   }
   if (symbol.owner().IsDerivedType() &&
       (symbol.attrs().test(Attr::CONTIGUOUS) &&

flang/test/Semantics/declarations01.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+! test named constant declarations
+
+function f1() result(x)
+  !ERROR: A function result may not also be a named constant
+  integer, parameter :: x = 1
+end
+