FIR checker: differentiate UNSAFE_CALL from INAPPLICABLE_CANDIDATE

To do so, inside the root cause of inapplicable candidate errors,
we will record expected/actual type of receiver, if any.
That will help identifying inapplicable calls on nullable receiver.

compiler/testData/diagnostics/tests/inference/regressions/kt2842.fir.kt

@@ -3,14 +3,14 @@ package c
 interface A<T>
 
 fun test(a: A<Int>?) {
-    a.<!INAPPLICABLE_CANDIDATE!>foo<!>() //no error
+    a.<!UNSAFE_CALL!>foo<!>() //no error
 }
 
 fun <R> A<R>.foo() {}
 
 //------------
 fun test(nullabilityInfoMap: Map<Int, Any>?) {
-    nullabilityInfoMap.<!INAPPLICABLE_CANDIDATE!>iterator<!>() //no error
+    nullabilityInfoMap.<!UNSAFE_CALL!>iterator<!>() //no error
 }
 
 //resolves to
@@ -20,7 +20,7 @@ public fun <K,V> Map<K,V>.iterator(): Iterator<Map.Entry<K,V>> {}
 //-------------
 fun foo() : Boolean {
     val nullableList = getNullableList()
-    return nullableList.<!INAPPLICABLE_CANDIDATE!>contains<!>("")
+    return nullableList.<!UNSAFE_CALL!>contains<!>("")
 
 }