Add FuzzedDataProvider helper class / single header library.

Summary:
This class is useful for writing fuzz target that have multiple inputs.

Current CL imports the existing `FuzzedDataProvider` from Chromium
without any modifications. Feel free to review it thoroughly, if you're
interested, but I'd prefer changing the class in a follow up CL.

The CL also introduces an exhaustive test for the library, as the behavior
of `FuzzedDataProvider` must not change over time.

In follow up CLs I'm planning on changing some implementation details
(I can share a doc with some comments to be addressed). After that, we
will document how `FuzzedDataProvider` should be used.

I have tested this on Linux, Windows and Mac platforms.

Reviewers: morehouse, metzman, kcc

Reviewed By: morehouse

Subscribers: metzman, thakis, rnk, mgorny, ormris, delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D62733

llvm-svn: 363071