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/smartCasts/loops/whileWithAssertInConditionAndBreakAfter.fir.kt

@@ -8,7 +8,7 @@ fun foo() {
 
     }
     // TODO: this testdata fixates undesired behavior (it should be an unsafe call)
-    x.<!INAPPLICABLE_CANDIDATE!>length<!> // 'x' is unsoundly smartcasted here
+    x.<!UNSAFE_CALL!>length<!> // 'x' is unsoundly smartcasted here
 }
 
 fun bar() {
@@ -19,5 +19,5 @@ fun bar() {
 
     }
     // TODO: this testdata fixates undesired behavior (it should be an unsafe call)
-    x.<!INAPPLICABLE_CANDIDATE!>size<!> // 'x' is unsoundly smartcasted here
+    x.<!UNSAFE_CALL!>size<!> // 'x' is unsoundly smartcasted here
 }