Defining a new API for debug options that doesn't rely on static global cl::opts.

Summary:
This is based on the discussions from the LLVMDev thread:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-August/075886.html

Reviewers: chandlerc

Reviewed By: chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5389

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219854 91177308-0d34-0410-b5e6-96231b3b80d8