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/functionalBound.fir.kt

@@ -1,6 +1,6 @@
 fun <E : String?, T : ((CharSequence) -> Unit)?> foo(x: E, y: T) {
     if (x != null) {
-        y(x)
+        <!INAPPLICABLE_CANDIDATE!>y<!>(x)
     }
 
     if (y != null) {
@@ -10,4 +10,4 @@ fun <E : String?, T : ((CharSequence) -> Unit)?> foo(x: E, y: T) {
     if (x != null && y != null) {
         y(x)
     }
-}
+}