FIR: Make LHS of FirVariableAssignment a FirExpression

This way references can even be resolved for erroneous assignments
(e.g. function call, if expression, ... on LHS)

^KT-54648 Fixed

compiler/frontend.common/src/org/jetbrains/kotlin/KtSourceElement.kt

@@ -240,6 +240,9 @@ sealed class KtFakeSourceElementKind : KtSourceElementKind() {
 
     // for all implicit receivers (now used for qualifiers only)
     object ImplicitReceiver : KtFakeSourceElementKind()
+
+    // for when on the LHS of an assignment an error expression appears
+    object AssignmentLValueError : KtFakeSourceElementKind()
 }
 
 sealed class AbstractKtSourceElement {