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