Adding a new cl::HideUnrelatedOptions API to allow clang to migrate off cl::getRegisteredOptions.

Summary: cl::getRegisteredOptions really exposes some of the innards of how command line parsing is implemented. Exposing new APIs that allow us to disentangle client code from implementation details will allow us to make more extensive changes to command line parsing.

Reviewers: chandlerc, dexonsmith, beanz

Reviewed By: dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7100

llvm-svn: 226729