The old 'FirProviderInterceptor' is too invasive, ad-hoc, and only a
few places in the compiler are aware of it.
This fixes the test 'FirOnAirResolveTestGenerated#testOnAirTypesResolve'
which started failing after the designation path collection refactoring.
The newly appeared class existence check started to fail:
```
val declaration = firProvider.getFirClassifierByFqName(classId)
check(declaration != null)
```
Yan Zhulanow
e5cbe79027