[CMake] Accept entitlements for code signing in add_lldb_library()

Summary:
D57334 added entitlements support in `add_llvm_library()` so we can use it for library targets in LLDB.

Additionally this patch fixes the way that the entitlements argument is passed on from `add_lldb_executable()` to `add_llvm_executable()`. We still need the explicit parsing and passing on of single- and multi-value arguments as long as we are on CMake < 3.7 (due to bug https://gitlab.kitware.com/cmake/cmake/merge_requests/133).

Reviewers: beanz, JDevlieghere, aprantl

Reviewed By: JDevlieghere

Subscribers: mgorny, lldb-commits, #lldb

Differential Revision: https://reviews.llvm.org/D57378

llvm-svn: 352629