[IR] Support isAssignable in builders and serialization.

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/builders/declarations/IrValueParameterBuilder.kt

@@ -18,6 +18,7 @@ class IrValueParameterBuilder : IrDeclarationBuilder() {
     var isCrossInline = false
     var isNoinline = false
     var isHidden = false
+    var isAssignable = false
 
     fun updateFrom(from: IrValueParameter) {
         super.updateFrom(from)
@@ -28,5 +29,6 @@ class IrValueParameterBuilder : IrDeclarationBuilder() {
         isCrossInline = from.isCrossinline
         isNoinline = from.isNoinline
         isHidden = from.isHidden
+        isAssignable = from.isAssignable
     }
 }

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/impl/IrSetValueImpl.kt

@@ -16,6 +16,7 @@
 
 package org.jetbrains.kotlin.ir.expressions.impl
 
+import org.jetbrains.kotlin.ir.declarations.IrValueParameter
 import org.jetbrains.kotlin.ir.expressions.IrExpression
 import org.jetbrains.kotlin.ir.expressions.IrSetValue
 import org.jetbrains.kotlin.ir.expressions.IrStatementOrigin
@@ -32,6 +33,24 @@ class IrSetValueImpl(
     override var value: IrExpression,
     override val origin: IrStatementOrigin?
 ) : IrSetValue() {
+
+    companion object {
+        private fun isVariableOrAssignable(symbol: IrValueSymbol): Boolean {
+            val declaration = symbol.owner
+            return if (declaration is IrValueParameter) {
+                declaration.isAssignable
+            } else {
+                true
+            }
+        }
+    }
+
+    init {
+        assert(isVariableOrAssignable(symbol)) {
+            "Only IrVariables and assignable IrValueParameters can be set"
+        }
+    }
+
     override fun <R, D> accept(visitor: IrElementVisitor<R, D>, data: D): R {
         return visitor.visitSetValue(this, data)
     }

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DeepCopyIrTreeWithSymbols.kt

@@ -349,7 +349,7 @@ open class DeepCopyIrTreeWithSymbols(
             declaration.varargElementType?.remapType(),
             declaration.isCrossinline,
             declaration.isNoinline,
-            false
+            declaration.isAssignable
         ).apply {
             transformAnnotations(declaration)
             defaultValue = declaration.defaultValue?.transform()

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/RenderIrElement.kt

@@ -525,6 +525,7 @@ class RenderIrElementVisitor(private val normalizeNames: Boolean = false) : IrEl
             "vararg".takeIf { varargElementType != null },
             "crossinline".takeIf { isCrossinline },
             "noinline".takeIf { isNoinline },
+            "assignable".takeIf { isAssignable }
         )
 
     override fun visitLocalDelegatedProperty(declaration: IrLocalDelegatedProperty, data: Nothing?): String =