Add conversion folding when the source is a constant.

This change adds unary conversion folding when the source is a constant.
This fixes an ISV issue whereby:

```
const float16_t f = float16_t(42.0);
```

Wouldn't compile because the conversion operator would always produce an
EvqTemporary when it could have produced an EvqConst.

I've also added a test case that proves out that all basic-type to
basic-type conversions work.