[clang] Fix a cmake error when using the Xcode generator. (PR #119403)

[Paul Bowen-Huggett via cfe-commits]

paulhuggett wrote:

> I guess I must of broke build for the MSVC MSBuild/vcxproj CMake generator

(Thanks for the reply, and apologies for the slow response.) I've spent time trying to properly understand the flow through the add_clang_library and add_llvm_library. As far as I can tell, the implementation for the MSVC generator is just fine (at least when running cmake without any fancy options). The thing that is missing for Xcode is a test for the existence of the "${name}.obj" library that matches line #114 (as of commit 97ff961) in the MSVC branch of the if/elseif.

https://github.com/llvm/llvm-project/pull/119403