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/fir/analysis-tests/testData/resolveWithStdlib/plusAssignNullable.kt

@@ -0,0 +1,16 @@
+interface A {
+    val list: List<String>
+}
+
+interface B {
+    val list: MutableList<String>
+}
+
+fun B.foo(a: A?) {
+    list.plusAssign(mutableListOf(""))
+    with(a) {
+        list.plusAssign(mutableListOf(""))
+        list += mutableListOf("")
+    }
+}
+