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/testWithModifiedMockJdk/notConsideredMethod.fir.kt

@@ -7,7 +7,7 @@ interface A : MutableCollection<String> {
 }
 
 fun foo(x: MutableCollection<Int>, y: Collection<String>, z: A) {
-    x.<!UNRESOLVED_REFERENCE!>nonExistingMethod<!>(1).<!INAPPLICABLE_CANDIDATE!>checkType<!> { <!INAPPLICABLE_CANDIDATE!>_<!><String>() }
+    x.<!UNRESOLVED_REFERENCE!>nonExistingMethod<!>(1).<!UNSAFE_CALL!>checkType<!> { <!INAPPLICABLE_CANDIDATE!>_<!><String>() }
     y.<!UNRESOLVED_REFERENCE!>nonExistingMethod<!>("")
     z.nonExistingMethod("")
 }