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/generics/nullability/smartCastRefinedClass.fir.kt

@@ -1,6 +1,6 @@
 fun <T : Any?> foo(x: T) {
     if (x is String?) {
-        x.length
+        x.<!INAPPLICABLE_CANDIDATE!>length<!>
 
         if (x != null) {
             x.length