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/tests-spec/testData/diagnostics/notLinked/dfa/neg/30.fir.kt

@@ -30,8 +30,8 @@ class Case5(val y: Any?): ClassWithCostructorParam(y as Interface1), Interface1
 
 // TESTCASE NUMBER: 6
 fun case_6(a: Int?) = object : ClassWithCostructorParam(a!!) {
-    fun run() = a.<!INAPPLICABLE_CANDIDATE!>toShort<!>()
+    fun run() = a.<!UNSAFE_CALL!>toShort<!>()
     init {
-        println(a.<!INAPPLICABLE_CANDIDATE!>toShort<!>())
+        println(a.<!UNSAFE_CALL!>toShort<!>())
     }
 }