Previously `FirPropertyAccessor.source` references the `KtProperty` if
the user code contains a setter or getter that doesn't contain a body.
For example, with the following
```
val i: Int = 1
private set
```
The `FirPropertyAccessor` would reference the `KtProperty` as the
source.
This change makes `FirPropertyAccessor` reference `KtPropertyAccessor`
as the source if possible, regardless of whether the body is present or
not.
Jinseong Jeon