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/j+k/sam/enhancedSamConstructor.fir.kt

@@ -17,12 +17,12 @@ public interface J2 extends J {
 // FILE: main.kt
 fun main() {
     <!INAPPLICABLE_CANDIDATE!>J<!> { s: String -> s} // should be prohibited, because SAM value parameter has nullable type
-    J { "" + it.<!INAPPLICABLE_CANDIDATE!>length<!> }
+    J { "" + it.<!UNSAFE_CALL!>length<!> }
     J { null }
     J { it?.length?.toString() }
 
     <!INAPPLICABLE_CANDIDATE!>J2<!> { s: String -> s}
-    J2 { "" + it.<!INAPPLICABLE_CANDIDATE!>length<!> }
+    J2 { "" + it.<!UNSAFE_CALL!>length<!> }
     J2 { null }
     J2 { it?.length?.toString() }
 }