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/findAnnotation.kt

@@ -1,9 +1,12 @@
 // IGNORE_BACKEND: JS_IR
 // IGNORE_BACKEND: JS_IR_ES6
 // IGNORE_BACKEND: JS, NATIVE
+// !USE_EXPERIMENTAL: kotlin.ExperimentalStdlibApi
 // WITH_REFLECT
 
 import kotlin.reflect.full.findAnnotation
+import kotlin.reflect.full.findAnnotations
+import kotlin.test.assertEquals
 import kotlin.test.assertNull
 
 annotation class Yes(val value: String)
@@ -19,5 +22,7 @@ fun box(): String {
     assertNull(Bar::class.findAnnotation<Yes>())
     assertNull(Bar::class.findAnnotation<No>())
 
+    assertEquals("OK", Foo::class.findAnnotations<Yes>().single().value)
+
     return Foo::class.findAnnotation<Yes>()?.value ?: "Fail: no annotation"
 }