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)

idea/idea-frontend-fir/idea-fir-low-level-api/src/org/jetbrains/kotlin/idea/fir/low/level/api/resolver/SingleCandidateResolver.kt

@@ -68,7 +68,11 @@ class SingleCandidateResolver(
             resolutionParameters.callableSymbol,
             explicitReceiverKind = explicitReceiverKind,
             dispatchReceiverValue = dispatchReceiverValue,
-            implicitExtensionReceiverValue = implicitExtensionReceiverValue,
+            extensionReceiverValue =
+            if (explicitReceiverKind.isExtensionReceiver)
+                callInfo.explicitReceiver?.let { ExpressionReceiverValue(it) }
+            else
+                implicitExtensionReceiverValue,
             scope = null,
         )