[CMake] llvm_codesign workaround for Xcode double-signing errors
When using Xcode to build LLVM with code signing, the post-build rule is executed even if the actual build-step was skipped. This causes double-signing errors. We can currently only avoid it by passing the --force flag.
Plus some polishing for my previous patch D54443.
Reviewers: beanz, kubamracek
Reviewed By: kubamracek
Subscribers: #lldb, mgorny, llvm-commits
Differential Revision: https://reviews.llvm.org/D55116