[CMake] Fix Xcode build with LLVM_ENABLE_OBJLIB.

[CMake] Fix Xcode build with LLVM_ENABLE_OBJLIB.

This amends chapuni's r246156 to handle an Xcode quirk, one even called out
in the CMake documentation:

    Some native build systems may not like targets that have only object files,
    so consider adding at least one real source file to any target that
    references $<TARGET_OBJECTS:objlib>.

I've limited the scope of this hack to Xcode for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247305 91177308-0d34-0410-b5e6-96231b3b80d8