Hi,

This patch fixes an incorrect warning for the oacc copy clause.

Consider this test-case:
...
void
foo (void)
{
   int i;

#pragma acc kernels
   {
     i = 1;
   }
}
...


When compiling with -fopenacc -Wuninitialized, we get an 'is used 
uninitialized' warning for variable 'i', which is confusing given that 
'i' is not used, but only set in the kernels region.

The warning occurs because there's an implicit copy(i) clause on the 
kernels region, and that copy generates a read of i before the region, 
and a write to i in region.

The patch silences the warning by marking the variable in the copy 
clause with TREE_NO_WARNING.

Build and reg-tested with goacc.exp, gomp.exp and target-libgomp.

OK for trunk if bootstrap and reg-test succeeds?

Thanks,
- Tom