add prefix with '_' support for property name. Corresponding apple dev doc:...

add prefix with '_' support for property name. Corresponding apple dev doc: https://developer.apple.com/library/content/qa/qa1908/_index.html

Reviewers: benhamilton, hokein

Reviewed By: benhamilton

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D42464

llvm-svn: 323722