A wrapper block was introduced as a part of fixing KT-59748, but was
assigned a real source, which had `getOrBuildFir` confused because
of the incorrectly built `KtToFirMapping`
It is relevant for:
- `if`, `when` expressions with an assignment as a single expression
- Kotlin code fragments,
when the assignment is being analysed as a single expression
in the fragment
^KT-66411 Fixed
Use-site substitution override happens in situations like this:
```
interface List<A> { fun get(i: Int): A }
fun take(list: List<String>) {
list.get(10) // this call
}
```
We want to have those overrides unwrapped, because we don't want
to deal with a different KtSymbol for each possible use-site
^KT-50862 Fixed
Roman Golyshev