[lldb] Import ObjectiveC module instead of Foundation in test (NFC)

[kastiglione]

Summary

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

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

class TestClangModuleAppUpdate(TestBase):
     @skipIf(debug_info=no_match(["gmodules"]))
     def test_rebuild_app_modules_untouched(self):
         with open(self.getBuildArtifact("module.modulemap"), "w") as f:
             f.write("""
                     module Foo { header "f.h" }
                     """)
         with open(self.getBuildArtifact("f.h"), "w") as f:
             f.write("""
-                    @import Foundation;
+                    @import ObjectiveC;
                     @interface Foo : NSObject {
                        int i;
                     }
                     +(instancetype)init;
                     @end
                     """)
 
         mod_cache = self.getBuildArtifact("private-module-cache")

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

-@import Foundation;
+@import ObjectiveC;
 @import Foo;
 @implementation Foo
 +(instancetype)init {
   return [super init];
 }
 @end