Random Number Generator (llvm)

Provides an abstraction for a random number generator (RNG) that produces a stream of pseudo-random numbers.
The current implementation uses C++11 facilities and is therefore not cryptographically secure.

The RNG is salted with the text of the current command line invocation.
In addition, a user may specify a seed (reproducible builds).

In clang, the seed can be set via

-frandom-seed=X
In the back end, the seed can be set via

-rng-seed=X
This is the llvm part of the patch.
clang part: D3391

URL: http://reviews.llvm.org/D3390
Author: yln

I'm landing this for the second time, it broke Windows bots the first time around.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211705 91177308-0d34-0410-b5e6-96231b3b80d8