[flang] Fix check for PRIVATE override of PUBLIC t.b.p.

[klausler]

Summary

A PRIVATE procedure binding in a derived type extension may not
be an override of a PUBLIC procedure binding. Declaration checking
for this case was working only in the presence of an explicit
PUBLIC accessibility attribute, when it should be checking for the
absence of a PRIVATE accessibility attribute.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-declarations.cpp

if (const auto *overriddenBinding{
             }
           }
         }
       }
       if (symbol.attrs().test(Attr::PRIVATE)) {
         if (FindModuleContaining(dtScope) ==
             FindModuleContaining(overridden->owner())) {
           // types declared in same madule
-          if (overridden->attrs().test(Attr::PUBLIC)) {
+          if (!overridden->attrs().test(Attr::PRIVATE)) {
             SayWithDeclaration(*overridden,
                 "A PRIVATE procedure may not override a PUBLIC procedure"_err_en_US);
           }
         } else { // types declared in distinct madules
           if (!CheckAccessibleSymbol(dtScope.parent(), *overridden)) {
             SayWithDeclaration(*overridden,
                 "A PRIVATE procedure may not override an accessible procedure"_err_en_US);
           }

flang/test/Semantics/bindings01.f90

contains
     procedure, private :: tbp => sub2
   end type
  contains
   subroutine sub2(x)
     class(t2), intent(in) :: x
   end subroutine
 end module
 
+module m11
+  type t1
+   contains
+    procedure, nopass :: tbp => t1p
+  end type
+  type, extends(t1) :: t2
+   contains
+    private
+    !ERROR: A PRIVATE procedure may not override a PUBLIC procedure
+    procedure, nopass :: tbp => t2p
+  end type
+ contains
+  subroutine t1p
+  end
+  subroutine t2p
+  end
+end
+
 program test
   use m1
   type,extends(t) :: t2
   end type
   type(t2) a
   call a%tbp
 end program