Introduce FuzzMutate library

This introduces the FuzzMutate library, which provides structured
fuzzing for LLVM IR, as described in my [EuroLLVM 2017 talk][1]. Most
of the basic mutators to inject and delete IR are provided, with
support for most basic operations.

I will follow up with the instruction selection fuzzer, which is
implemented in terms of this library.

[1]: http://llvm.org/devmtg/2017-03//2017/02/20/accepted-sessions.html#2

llvm-svn: 311356