Native: correct ObjCExport test for forward declarations

The test relies on the fact that any name can be imported from a "magic"
package like `objcnames.classes`.
This was the case for K1, but likely won't be for K2: the compiler
should allow importing only those names that are actually
forward-declared in a cinterop library.

This commit changes the test that way. In particular, moves it to a
different task that has a cinterop dependency.

kotlin-native/backend.native/tests/framework/forwardDeclarations/clib.def

@@ -0,0 +1,17 @@
+language=Objective-C
+---
+struct ForwardDeclaredStruct;
+@class ForwardDeclaredClass;
+@protocol ForwardDeclaredProtocol;
+
+struct ForwardDeclaredStruct* sameStruct(struct ForwardDeclaredStruct* ptr) {
+    return ptr;
+}
+
+ForwardDeclaredClass* sameClass(ForwardDeclaredClass* obj) {
+    return obj;
+}
+
+id<ForwardDeclaredProtocol> sameProtocol(id<ForwardDeclaredProtocol> obj) {
+    return obj;
+}

kotlin-native/backend.native/tests/framework/forwardDeclarations/lib.kt

@@ -0,0 +1,9 @@
+import clib.*
+import cnames.structs.ForwardDeclaredStruct
+import objcnames.classes.ForwardDeclaredClass
+import objcnames.protocols.ForwardDeclaredProtocolProtocol
+import kotlinx.cinterop.CPointer
+
+fun sameForwardDeclaredStruct(ptr: CPointer<ForwardDeclaredStruct>?): CPointer<ForwardDeclaredStruct>? = sameStruct(ptr)
+fun sameForwardDeclaredClass(obj: ForwardDeclaredClass?): ForwardDeclaredClass? = sameClass(obj)
+fun sameForwardDeclaredProtocol(obj: ForwardDeclaredProtocolProtocol?): ForwardDeclaredProtocolProtocol? = sameProtocol(obj)

kotlin-native/backend.native/tests/framework/forwardDeclarations/test.swift

@@ -0,0 +1,25 @@
+import ForwardDeclarations
+
+private func test1() throws {
+    let ptr = UnsafeMutableRawPointer(bitPattern: 0x1234)
+    try assertEquals(actual: LibKt.sameForwardDeclaredStruct(ptr: ptr), expected: ptr)
+
+    // We can't actually test this, because Swift can't import neither types nor functions due to
+    // "interface/protocol '...' is incomplete":
+    //
+    // let classObj: ForwardDeclaredClass? = nil
+    // try assertNil(LibKt.sameForwardDeclaredClass(obj: classObj))
+    //
+    // let protocolObj: ForwardDeclaredProtocol? = nil
+    // try assertNil(LibKt.sameForwardDeclaredProtocol(obj: protocolObj))
+}
+
+// -------- Execution of the test --------
+
+class TestTests : SimpleTestProvider {
+    override init() {
+        super.init()
+
+        test("test1", test1)
+    }
+}