[lldb] Replace std::once_flag with llvm::once_flag.

Summary:
The former seems like it's not working on some platforms.
All the other uses use `llvm::`, so, let's change for consistency.

Reviewers: jasonmolenda, friss

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D66566

llvm-svn: 369618