K2: use Any? as last argument when serializing suspend types

K1 uses `Any?` in `transformSuspendFunctionToRuntimeFunctionType`, so
use `Any?` in K2 too.

This type is not used for anything, it's just an artifact of how suspend
types are represented in metadata. It's removed during deserialization
(see `TypeDeserializer.createSuspendFunctionType` or
`FirTypeDeserializer.createSuspendFunctionType`). So it's not a
user-visible change. The only effect is that it reduces differences in
metadata between K1 and K2, namely in the test
`libraries/tools/kotlinp/testData/ValueClass.kt`.

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/serializationUtil.kt

@@ -41,7 +41,7 @@ fun ConeKotlinType.suspendFunctionTypeToFunctionTypeWithContinuation(session: Fi
         typeArguments = (typeArguments.dropLast(1) + continuationClassId.toLookupTag().constructClassType(
             arrayOf(lastTypeArgument),
             isNullable = false
-        ) + lastTypeArgument).toTypedArray(),
+        ) + session.builtinTypes.nullableAnyType.type).toTypedArray(),
         isNullable = fullyExpandedType.isNullable,
         attributes = fullyExpandedType.attributes
     )
@@ -49,4 +49,4 @@ fun ConeKotlinType.suspendFunctionTypeToFunctionTypeWithContinuation(session: Fi
 
 fun FirMemberDeclaration.shouldBeSerialized(actualizedExpectDeclaration: Set<FirDeclaration>?): Boolean {
     return !isExpect || actualizedExpectDeclaration == null || this !in actualizedExpectDeclaration
-}
+}