[IR] Move IrElement#sourceElement to the ir.tree module

This is so that it could be used from more places.
Namely, we need to use it from the `serialization.common` module.

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

@@ -5,6 +5,8 @@
 
 package org.jetbrains.kotlin.ir.util
 
+import org.jetbrains.kotlin.AbstractKtSourceElement
+import org.jetbrains.kotlin.KtOffsetsOnlySourceElement
 import org.jetbrains.kotlin.builtins.PrimitiveType
 import org.jetbrains.kotlin.builtins.StandardNames
 import org.jetbrains.kotlin.builtins.UnsignedType
@@ -1610,4 +1612,8 @@ val IrDeclaration.parents: Sequence<IrDeclarationParent>
 val IrDeclaration.isExpect
     get() = this is IrClass && isExpect ||
             this is IrFunction && isExpect ||
-            this is IrProperty && isExpect
+            this is IrProperty && isExpect
+
+fun IrElement.sourceElement(): AbstractKtSourceElement? =
+    if (startOffset >= 0) KtOffsetsOnlySourceElement(this.startOffset, this.endOffset)
+    else null