[flang] Don't check procedure pointer interface function result specification expressions

[klausler]

Summary

When a procedure pointer references a function as its interface, don't
apply semantic checks to the specification expressions that appear in
the declaration of the function's result -- this can lead to bogus
error messages as those specification expressions are being examined
out of their proper context.

flang/lib/Semantics/check-declarations.cpp

if (!IsStmtFunctionDummy(symbol)) { // C726
         canHaveAssumedParameter |= object->isDummy() ||
             (object->isFuncResult() &&
                 type->category() == DeclTypeSpec::Character) ||
             IsStmtFunctionResult(symbol); // Avoids multiple messages
       } else {
         canHaveAssumedParameter |= symbol.has<AssocEntityDetails>();
       }
     }
+    if (IsProcedurePointer(symbol) && symbol.HasExplicitInterface()) {
+      // Don't check function result types here
+    } else {
       Check(*type, canHaveAssumedParameter);
+    }
     if (InPure() && InFunction() && IsFunctionResult(symbol)) {
       if (derived && HasImpureFinal(*derived)) { // C1584
         messages_.Say(
             "Result of pure function may not have an impure FINAL subroutine"_err_en_US);
       }
       if (type->IsPolymorphic() && IsAllocatable(symbol)) { // C1585
         messages_.Say(
             "Result of pure function may not be both polymorphic and ALLOCATABLE"_err_en_US);