Support new repeatable annotations in kotlin-reflect

- Unwrap Kotlin-repeatable annotations (with implicit container)
- Introduce `KAnnotatedElement.findAnnotations` to find instances of
  repeated annotations

 #KT-12794

compiler/testData/codegen/box/reflection/annotations/repeatable/jvmRepeatableKotlinAnnotation.kt

@@ -0,0 +1,36 @@
+// !LANGUAGE: +RepeatableAnnotations
+// !USE_EXPERIMENTAL: kotlin.ExperimentalStdlibApi
+// TARGET_BACKEND: JVM_IR
+// JVM_TARGET: 1.8
+// FULL_JDK
+// WITH_REFLECT
+
+// Android doesn't have @Repeatable, so findAnnotations can't unpack repeatable annotations.
+// IGNORE_BACKEND: ANDROID
+
+import kotlin.reflect.full.findAnnotation
+import kotlin.reflect.full.findAnnotations
+import kotlin.reflect.full.hasAnnotation
+
+@java.lang.annotation.Repeatable(A.Container::class)
+annotation class A(val value: String) {
+    annotation class Container(val value: Array<A>)
+}
+
+@A("O")
+@A("")
+@A("K")
+fun f() {}
+
+fun box(): String {
+    val element = ::f
+    if (element.hasAnnotation<A>()) return "Fail hasAnnotation $element"
+    val find = element.findAnnotation<A>()
+    if (find != null) return "Fail findAnnotation $element: $find"
+
+    val all = (element.annotations.single() as A.Container).value.asList()
+    val findAll = element.findAnnotations<A>()
+    if (all != findAll) throw AssertionError("Fail findAnnotations $element: $all != $findAll")
+
+    return all.fold("") { acc, it -> acc + it.value }
+}