Add support for KSBackingField in ModifierVisitor

kotlin-analysis-api/src/test/kotlin/com/google/devtools/ksp/processor/JavaModifierProcessor.kt

diff --git a/kotlin-analysis-api/src/test/kotlin/com/google/devtools/ksp/processor/JavaModifierProcessor.kt b/kotlin-analysis-api/src/test/kotlin/com/google/devtools/ksp/processor/JavaModifierProcessor.kt
index 53c8263..3b54987 100644
--- a/kotlin-analysis-api/src/test/kotlin/com/google/devtools/ksp/processor/JavaModifierProcessor.kt
+++ b/kotlin-analysis-api/src/test/kotlin/com/google/devtools/ksp/processor/JavaModifierProcessor.kt
@@ -70,6 +70,11 @@
 
         override fun visitPropertyDeclaration(property: KSPropertyDeclaration, data: Unit) {
             results.add(property.toSignature())
+            property.backingField?.accept(this, data)
+        }
+
+        override fun visitBackingField(backingField: KSBackingField, data: Unit) {
+            results.add(backingField.toSignature())
         }
 
         override fun visitFunctionDeclaration(function: KSFunctionDeclaration, data: Unit) {