Prohibit Array<T> in @Serializable classes

because it's impossible to correctly create array reflectively without
knowing correct KClass.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/1243

plugins/kotlinx-serialization/testData/diagnostics/GenericArrays.kt

@@ -0,0 +1,13 @@
+// FIR_IDENTICAL
+// WITH_STDLIB
+// SKIP_TXT
+import kotlinx.serialization.*
+
+@Serializable
+class C(val values: IntArray) // OK
+
+@Serializable
+class B(val values: Array<String>) // OK
+
+@Serializable
+class A<T>(val values: <!GENERIC_ARRAY_ELEMENT_NOT_SUPPORTED!>Array<T><!>)