JS: cache PropertyMetadata objects in property delegates

Also, remove unused PropertyMetadata instances

See KT-20737, KT-20738

js/js.translator/testData/box/delegateProperty/metadataReferentialEquality.kt

@@ -0,0 +1,40 @@
+// EXPECTED_REACHABLE_NODES: 1144
+import kotlin.reflect.KProperty
+
+var lastGeneratedId = 0
+val idMap = mutableMapOf<KProperty<*>, Int>()
+
+class MyDelegate(val value: String) {
+    operator fun getValue(receiver: Any?, property: KProperty<*>): String {
+        val id = idMap.getOrPut(property) { lastGeneratedId++ }
+        return "${property.name}:$value:$id"
+    }
+}
+
+class C {
+    val foo by MyDelegate("C")
+}
+
+val bar by MyDelegate("toplevel")
+
+fun box(): String {
+    val c = C()
+
+    var a = c.foo
+    var b = c.foo
+    if (a !== b) return "fail: member property referential equality"
+    if (!a.startsWith("foo:C:")) return "fail: member property value"
+
+    a = bar
+    b = bar
+    if (a !== b) return "fail: top level property referential equality"
+    if (!a.startsWith("bar:toplevel:")) return "fail: top level property value"
+
+    val baz by MyDelegate("local")
+    a = baz
+    b = baz
+    if (a !== b) return "fail: local property referential equality"
+    if (!a.startsWith("baz:local:")) return "fail: local property value"
+
+    return "OK"
+}

js/js.translator/testData/box/delegateProperty/unusedPropertyMetadata.kt

@@ -0,0 +1,12 @@
+// EXPECTED_REACHABLE_NODES: 1131
+// PROPERTY_NOT_USED: PropertyMetadata
+import kotlin.reflect.KProperty
+
+class MyDelegate(val value: String) {
+    inline operator fun getValue(receiver: Any?, property: KProperty<*>): String = value
+}
+
+fun box(): String {
+    val x by MyDelegate("OK")
+    return x
+}