[callables] Overwriting dtypes at a call-site should be illegal

The following translation unit:
```python
alpha_set = lp.make_function(
"{[i]: 0<=i<10}",
"""
y[i] = alpha
""", [lp.ValueArg("alpha", dtype=np.float64),
...],
name="alpha_set")
knl = lp.make_kernel(
"{[i]: 0<=i<10}",
"""
<>tmp = 3.14f
[i]: Y[i] = alpha_set(tmp)
""")
```

changes the `dtype` of `alpha` in the kernel `alpha_set` to `np.float32`.