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/safecalls/chainMixedUnsafe.fir.kt

@@ -1,5 +1,5 @@
 fun calc(x: List<String>?): Int {
     // x is not-null only inside subList
-    x?.subList(0, x.size - 1).<!INAPPLICABLE_CANDIDATE!>get<!>(x.<!INAPPLICABLE_CANDIDATE!>size<!>)
+    x?.subList(0, x.size - 1).<!UNSAFE_CALL!>get<!>(x.<!UNSAFE_CALL!>size<!>)
     return x!!.size
 }