[CMake] Use XCODE_ATTRIBUTE properties for code signing and entitlements in Xcode

Summary: A post-commit comment to D55116 amended that this was the correct way for code signing in Xcode.

Reviewers: beanz

Reviewed By: beanz

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D55816

llvm-svn: 350383