Re-apply "Introduce FuzzMutate library"

Redo r311356 with a fix to avoid std::uniform_int_distribution<bool>.
The bool specialization is undefined according to the standard, even
though libc++ seems to have it.

Original commit message:

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.

llvm-svn: 311392