[CMake] Accept ENTITLEMENTS in llvm_add_library()

Summary: We added support for code signing entitlements in add_llvm_executable() with D54443. In the future it would be useful to have this functionality available also for libraries.

Reviewers: beanz, bogner

Reviewed By: bogner

Subscribers: mgorny, llvm-commits, lldb-commits, #lldb

Differential Revision: https://reviews.llvm.org/D57334

llvm-svn: 352628