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/elvisNothingRHS.fir.kt

@@ -4,7 +4,7 @@ class C {
 
 fun test(a: C?, nn: () -> Nothing?) {
     a ?: nn()
-    a.<!INAPPLICABLE_CANDIDATE!>foo<!>()
+    a.<!UNSAFE_CALL!>foo<!>()
 
     a ?: return
     a.foo()