[lldb] Import ObjectiveC module instead of Foundation in test

[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