[CMake] llvm_codesign workaround for Xcode double-signing errors

Summary:
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

llvm-svn: 349070