[FIR] Track local variable assigned types for smartcast stability

When a lambda captures a local variable without a contract, future
assignments can impact the smartcast stability for that variable within
the lambda. Because future assignments can appear in both resolved and
unresolved code, track each assignment as encountered. To determine if
a smartcast can be considered stable, check that all future assignments
are resolved and do not revert variable to a super type.

^KT-58191 Fixed

compiler/testData/diagnostics/tests/smartCasts/lambdasWithContracts/lambdaWithCallInPlace.fir.kt

@@ -55,7 +55,7 @@ fun test4() {
     require(x is String)
     runWithoutContract {
         x = ""
-        <!SMARTCAST_IMPOSSIBLE!>x<!>.length
+        x.length
     }
 }