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/callBreakFirstInsideDoWhile.fir.kt

@@ -7,7 +7,7 @@ public fun foo(x: String?, z: String?): Int {
         gav(if (x == null) break else x, z!!)
     } while (bar())
     // z is nullable despite of z!!
-    z.<!INAPPLICABLE_CANDIDATE!>length<!>
+    z.<!UNSAFE_CALL!>length<!>
     // x is null because of the break
-    return x.<!INAPPLICABLE_CANDIDATE!>length<!>
+    return x.<!UNSAFE_CALL!>length<!>
 }