Add support for -fsanitize-blacklist and default blacklists for DFSan.
Also add some documentation.

Differential Revision: http://llvm-reviews.chandlerc.com/D1346

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188403 91177308-0d34-0410-b5e6-96231b3b80d8