Allow value repeat operator on references

Allow value repeat operator on references

Currently it's not possible to use the value repeat operator on references:
```
print ((int &) v_int_array_init[0])@2
Only values in memory can be extended with '@'.
```

This seems like an unnecessary restriction, since it also prevents
its use on iterators (which was the original reported problem):
```
(gdb) p *it@2
Only values in memory can be extended with '@'.
```

So this converts any references to the referenced value in value_repeat,
making this possible:
```
print ((int &) v_int_array_init[0])@2
$1 = {10, 20}
(gdb) p *it@2
$2 = {1, 2}
```

Approved-by: Kevin Buettner <kevinb@redhat.com>