FIR: Rework receivers processing in resolution

- Put extensionReceiver to candidate even if it's explicit (for sake of clarity)
- Split CheckReceiver (dispatch part should only check nullability)

compiler/testData/diagnostics/tests/callableReference/bound/expressionWithNullableType.fir.kt

@@ -7,8 +7,8 @@ public interface J {
 // FILE: test.kt
 
 fun f1(x: Int?): Any = <!UNRESOLVED_REFERENCE!>x::hashCode<!>
-fun <T> f2(t: T): Any = t::hashCode
-fun <S : String?> f3(s: S): Any = s::hashCode
+fun <T> f2(t: T): Any = <!UNRESOLVED_REFERENCE!>t::hashCode<!>
+fun <S : String?> f3(s: S): Any = <!UNRESOLVED_REFERENCE!>s::hashCode<!>
 fun <U : Any> f4(u: U?): Any = <!UNRESOLVED_REFERENCE!>u::hashCode<!>
 fun f5(c: List<*>): Any = <!UNRESOLVED_REFERENCE!>c[0]::hashCode<!>