Fix declare copyout in libgomp.oacc-c++/declare-pr94120.C

Testing on the host does not make sense for 'declare copyout' for
a same-scope stack-allocated variable. Once the copyout is done,
the variable is gone. Hence, test the variable on the device. This
can be revisit after the OpenACC semantic has been fixed; but with
that fix, the test PASSes again with devices.

        PR middle-end/94120
        * testsuite/libgomp.oacc-c++/declare-pr94120.C: Fix 'declare copy(out)'
        test case.

libgomp/ChangeLog

@@ -1,3 +1,9 @@
+2020-04-20  Tobias Burnus  <tobias@codesourcery.com>
+
+	PR middle-end/94120
+	* testsuite/libgomp.oacc-c++/declare-pr94120.C: Fix 'declare copy(out)'
+	test case.
+
 2020-04-17  Tobias Burnus  <tobias@codesourcery.com>
 
 	PR middle-end/94635

libgomp/testsuite/libgomp.oacc-c++/declare-pr94120.C

@@ -35,6 +35,7 @@ f (void)
       C[i] = outer::inner::B[i];
     }
 
+#pragma acc parallel
   for (i = 0; i < N; i++)
     {
       if (C[i] != i + 1)