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/codegen/box/inlineClasses/toStringOfUnboxedNullable.kt

@@ -1,6 +1,5 @@
 // !LANGUAGE: +InlineClasses
 // IGNORE_BACKEND: JVM
-// IGNORE_BACKEND_FIR: JVM_IR
 // IGNORE_LIGHT_ANALYSIS
 inline class IC(val x: String)