[FIR] Introduce & use toFirLightSourceElement

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirModifierList.kt

@@ -65,6 +65,6 @@ val FirModifier<*>.source: FirSourceElement?
             // TODO pretty sure I got offsets wrong here
             val startOffset = tree.getStartOffset(node)
             val endOffset = tree.getEndOffset(node)
-            FirLightSourceElement(node, startOffset, endOffset, tree)
+            node.toFirLightSourceElement(startOffset, endOffset, tree)
         }
     }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirSourceElement.kt

@@ -49,6 +49,12 @@ val FirElement.psi: PsiElement? get() = (source as? FirPsiSourceElement<*>)?.psi
 @Suppress("NOTHING_TO_INLINE")
 inline fun PsiElement.toFirPsiSourceElement(): FirPsiSourceElement<*> = FirPsiSourceElement(this)
 
+@Suppress("NOTHING_TO_INLINE")
+inline fun LighterASTNode.toFirLightSourceElement(
+    startOffset: Int, endOffset: Int,
+    tree: FlyweightCapableTreeStructure<LighterASTNode>
+): FirLightSourceElement = FirLightSourceElement(this, startOffset, endOffset, tree)
+
 val FirSourceElement?.lightNode: LighterASTNode? get() = (this as? FirLightSourceElement)?.element
 
 fun FirSourceElement?.getModifierList(): FirModifierList? {