[IR] Remove custom error messages for missing IrDeclaration.parent

This is to remove custom logic from those classes,
so that source generator will be able to generate
them in a straight-forward way.
Otherwise, it would need to be thought about those special cases, which
is better to be avoided.

It is also a small step for future work,
which likely involves making parent property nullable.

Unfortunately, this change causes a loss of some details from exception
messages.

^KT-65773 In Progress

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrScriptImpl.kt

@@ -30,13 +30,7 @@ class IrScriptImpl(
 ) : IrScript() {
     override var origin: IrDeclarationOrigin = SCRIPT_ORIGIN
 
-    private var _parent: IrDeclarationParent? = null
-    override var parent: IrDeclarationParent
-        get() = _parent
-            ?: throw UninitializedPropertyAccessException("Parent not initialized: $this")
-        set(v) {
-            _parent = v
-        }
+    override lateinit var parent: IrDeclarationParent
 
     override var annotations: List<IrConstructorCall> = SmartList()
 

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrVariableImpl.kt

@@ -39,13 +39,7 @@ class IrVariableImpl(
     override var isConst: Boolean,
     override var isLateinit: Boolean
 ) : IrVariable() {
-    private var _parent: IrDeclarationParent? = null
-    override var parent: IrDeclarationParent
-        get() = _parent
-            ?: throw UninitializedPropertyAccessException("Parent not initialized: $this")
-        set(v) {
-            _parent = v
-        }
+    override lateinit var parent: IrDeclarationParent
 
     override var annotations: List<IrConstructorCall> = emptyList()