[lldb] Import ObjectiveC module instead of Foundation in test

Use `@import ObjectiveC` instead of `@import Foundation`, as the former is all
that's needed, and results in fewer clang modules being built.

This results in the following clang modules *not* being built for this test.

ApplicationServices
CFNetwork
ColorSync
CoreFoundation
CoreGraphics
CoreServices
CoreText
DiskArbitration
Dispatch
Foundation
IOKit
ImageIO
Security
XPC
_Builtin_intrinsics
launch
libkern
os_object
os_workgroup

Differential Revision: https://reviews.llvm.org/D99859

lldb/test/API/lang/objc/modules-app-update/TestClangModulesAppUpdate.py

@@ -20,7 +20,7 @@ class TestClangModuleAppUpdate(TestBase):
                     """)
         with open(self.getBuildArtifact("f.h"), "w") as f:
             f.write("""
-                    @import Foundation;
+                    @import ObjectiveC;
                     @interface Foo : NSObject {
                        int i;
                     }

lldb/test/API/lang/objc/modules-app-update/foo.m

@@ -1,4 +1,4 @@
-@import Foundation;
+@import ObjectiveC;
 @import Foo;
 @implementation Foo
 +(instancetype)init {