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/castchecks/basicOn.fir.kt

@@ -13,7 +13,7 @@ fun g(a: SomeClass?) {
         // 'a' can be cast to SomeSubClass
         a.hashCode()
         a.foo
-        (a as? SomeSubClass).<!INAPPLICABLE_CANDIDATE!>foo<!>
+        (a as? SomeSubClass).<!UNSAFE_CALL!>foo<!>
         (a as SomeSubClass).foo
     }
     val b = (a as? SomeSubClass)?.foo
@@ -21,7 +21,7 @@ fun g(a: SomeClass?) {
         // 'a' can be cast to SomeSubClass
         a.hashCode()
         a.foo
-        (a as? SomeSubClass).<!INAPPLICABLE_CANDIDATE!>foo<!>
+        (a as? SomeSubClass).<!UNSAFE_CALL!>foo<!>
         (a as SomeSubClass).foo
     }
     val c = a as? SomeSubClass
@@ -29,7 +29,7 @@ fun g(a: SomeClass?) {
         // 'a' and 'c' can be cast to SomeSubClass
         a.hashCode()
         a.foo
-        (a as? SomeSubClass).<!INAPPLICABLE_CANDIDATE!>foo<!>
+        (a as? SomeSubClass).<!UNSAFE_CALL!>foo<!>
         c.hashCode()
         c.foo
     }