Revamp llvm::once_flag to be closer to std::once_flag
Make this interface reusable similarly to std::call_once and std::once_flag interface.
This makes porting LLDB to NetBSD easier as there was in the original approach a portable way to specify a non-static once_flag. With this change translating std::once_flag to llvm::once_flag is mechanical.
Sponsored by <The NetBSD Foundation>
Reviewers: mehdi_amini, labath, joerg
Reviewed By: mehdi_amini
Subscribers: emaste, clayborg
Differential Revision: https://reviews.llvm.org/D29566