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/smartCastsOnThis.fir.kt

@@ -23,7 +23,7 @@ fun <T : String?> T.foo() {
         this?.bar1()
     }
 
-    length
+    <!INAPPLICABLE_CANDIDATE!>length<!>
 
     if (this is String) {
         length
@@ -33,4 +33,4 @@ fun <T : String?> T.foo() {
         bar2()
         bar3()
     }
-}
+}