Add DXIL Bitcode Writer and DXIL testing

This change is a big blob of code that isn't easy to break up. It
either comes in all together as a blob, works and has tests, or it
doesn't do anything.

Logically you can think of this patch as three things:
(1) Adding virtual interfaces so the bitcode writer can be overridden
(2) Adding a new bitcode writer implementation for DXIL
(3) Adding some (optional) crazy CMake goop to build the
DirectXShaderCompiler's llvm-dis as dxil-dis for testing

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D122082