[FIR] Rename isCastPossible to checkCasting, return CastingType instead of boolean

compiler/testData/diagnostics/tests/smartCasts/castchecks/variables.fir.kt

@@ -43,7 +43,7 @@ fun f(a: SomeClass?) {
         aa<!UNSAFE_CALL!>.<!>hashCode()
         aa.<!UNRESOLVED_REFERENCE!>foo<!>
         (aa as? SomeSubClass)<!UNSAFE_CALL!>.<!>foo
-        (aa <!CAST_NEVER_SUCCEEDS!>as<!> SomeSubClass).foo
+        (aa as SomeSubClass).foo
     }
     val b = (aa as? SomeSubClass)?.foo
     aa = null
@@ -52,7 +52,7 @@ fun f(a: SomeClass?) {
         aa<!UNSAFE_CALL!>.<!>hashCode()
         aa.<!UNRESOLVED_REFERENCE!>foo<!>
         (aa as? SomeSubClass)<!UNSAFE_CALL!>.<!>foo
-        (aa <!CAST_NEVER_SUCCEEDS!>as<!> SomeSubClass).foo
+        (aa as SomeSubClass).foo
     }
     aa = a
     val c = aa as? SomeSubClass