DFSan-based fuzzer (proof of concept).

Summary:
This adds a simple DFSan-based (i.e. taint-guided) fuzzer mutator,
see the comments for details.

Test Plan: a test added

Reviewers: samsonov, pcc

Reviewed By: samsonov, pcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8669

llvm-svn: 233613