Don't serialize expect declaration descriptors (#1307)

Thus fix producing .klib from multi-platform sources

backend.native/tests/build.gradle

@@ -1959,6 +1959,13 @@ task mpp1(type: RunStandaloneKonanTest) {
     flags = ['-tr', '-Xmulti-platform']
 }
 
+task mpp2(type: LinkKonanTest) {
+    goldValue = "C(arg=42)\ntrue\n1\nh\n"
+    source = "codegen/mpp/mpp2.kt"
+    lib = "codegen/mpp/libmpp2.kt"
+    flags = ['-Xmulti-platform']
+}
+
 task unit1(type: RunKonanTest) {
     goldValue = "First\nkotlin.Unit\n"
     source = "codegen/basics/unit1.kt"

backend.native/tests/codegen/mpp/libmpp2.kt

@@ -0,0 +1,16 @@
+expect class C {
+    constructor(arg: Any?)
+}
+
+actual data class C actual constructor(val arg: Any?) {
+
+}
+
+expect class T
+actual typealias T = C
+
+expect fun f(arg: Int): Int
+actual fun f(arg: Int) = arg
+
+expect var p: String
+actual var p: String = "p"

backend.native/tests/codegen/mpp/mpp2.kt

@@ -0,0 +1,8 @@
+fun main(args: Array<String>) {
+    println(C(42))
+    println(C(42) is T)
+    println(f(1))
+
+    p = "h"
+    println(p)
+}