Make llvm::call_once more convenient to reuse out of LLVM

Summary:
Currently users need to set call `using namespace llvm;`, with this change it's no longer needed.

Sponsored by <The NetBSD Foundation>

Reviewers: labath, emaste, joerg, clayborg, mehdi_amini

Reviewed By: mehdi_amini

Subscribers: chandlerc

Differential Revision: https://reviews.llvm.org/D29296

llvm-svn: 293902