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/nullabilityAndSmartCasts/ReceiverNullability.fir.kt

@@ -30,7 +30,7 @@ fun A.test2() {
 }
 
 fun A?.test3() {
-    foo() // error
+    <!INAPPLICABLE_CANDIDATE!>foo<!>() // error
     <!INAPPLICABLE_CANDIDATE!>bar<!>() // error
     buzz()