[Sanitizer] Use COMMON_FLAG macro to describe common runtime flags.

Summary:
Introduce a single place where we specify flag type, name, default
value, and description. This removes a large amount of boilerplate
and ensures we won't leave flags uninitialized.

Test Plan: regression test suite

Reviewers: kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6851

llvm-svn: 225239