FIR checkers: report SMARTCAST_IMPOSSIBLE

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/tower/CandidateApplicability.kt

@@ -14,8 +14,8 @@ enum class CandidateApplicability {
     INAPPLICABLE_MODIFIER, // no expected modifier (eg infix call on non-infix function)
     IMPOSSIBLE_TO_GENERATE, // access to outer class from nested
     RUNTIME_ERROR, // problems with visibility
-    UNSAFE_CALL, // receiver nullability doesn't match
-    MAY_THROW_RUNTIME_ERROR, // unstable smart cast
+    UNSAFE_CALL, // receiver or argument nullability doesn't match
+    UNSTABLE_SMARTCAST, // unstable smart cast
     CONVENTION_ERROR, // missing infix, operator etc
     RESOLVED_LOW_PRIORITY,
     RESOLVED_NEED_PRESERVE_COMPATIBILITY, // call resolved successfully, but using new features that changes resolve