FIR: introduce FirExpressionWithSmartcastToNull

This new kind of expression encompasses the nullability of the original
expression after null check (or equivalent `is Nothing?` check).

Unlike FirExpressionWithSmartcast, this expression won't be materialized
during conversion to backend IR. Also, Nothing? is discarded when
computing the intersection of possible types from smartcast info.
In that way, Nothing? is not used during resolution, while such
smartcast info is stored in it (and the expression kind itself).

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/throwsEffect.fir.kt

@@ -26,7 +26,7 @@ fun testSpilling(x: Any?) {
         myAssert(x is String)
         x.length
     }
-    x.<!UNRESOLVED_REFERENCE!>length<!>
+    x<!UNSAFE_CALL!>.<!>length
 }
 
 fun testAssertInIf(x: Any?) {