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/funInterface/funInterfaceConversionOnReceiver.fir.kt

@@ -5,5 +5,5 @@ fun interface Bar {
 operator fun Bar.plus(b: Bar): String = invoke() + b.invoke()
 
 fun box(): String {
-    return { "O" } + { "K" }
+    return { "O" } <!INAPPLICABLE_CANDIDATE!>+<!> { "K" }
 }