[FIR] Set source of smart-cast expressions automatically

#KT-55835

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt

@@ -452,6 +452,10 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
                 value = "smartcastStability == SmartcastStability.STABLE_VALUE"
                 withGetter = true
             }
+            default("source") {
+                value = "originalExpression.source?.fakeElement(KtFakeSourceElementKind.SmartCastExpression)"
+            }
+            useTypes(fakeElementImport, fakeSourceElementKindImport)
         }
 
         impl(resolvedNamedReference)

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/importables.kt

@@ -15,3 +15,6 @@ val resolvedDeclarationStatusImport = ArbitraryImportable("org.jetbrains.kotlin.
 val buildResolvedTypeRefImport = ArbitraryImportable("org.jetbrains.kotlin.fir.types.builder", "buildResolvedTypeRef")
 val constructClassTypeImport = ArbitraryImportable("org.jetbrains.kotlin.fir.types", "constructClassType")
 val toLookupTagImport = ArbitraryImportable("org.jetbrains.kotlin.fir.types", "toLookupTag")
+
+val fakeSourceElementKindImport = ArbitraryImportable("org.jetbrains.kotlin", "KtFakeSourceElementKind")
+val fakeElementImport = ArbitraryImportable("org.jetbrains.kotlin", "fakeElement")