Make llvm::call_once more convenient to reuse out of LLVM
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
Differential Revision: https://reviews.llvm.org/D29296