[LL FIR] FileStructure: support dangling modifier list in non-end position

The root cause of the exception is that we missed such an element, and
it led to unresolved declaration during iteration over file declarations

^KT-65562 Fixed

analysis/low-level-api-fir/testData/fileStructure/danglingAnnotationInMiddle.kt

@@ -0,0 +1,4 @@
+annotation class MyAnn/* ClassDeclarationStructureElement */
+
+@MyAnn/* DeclarationStructureElement */
+fin class Foo/* ClassDeclarationStructureElement */