Add custom executable (output object) format

[tschuett]

The newest member to the family is XCOFF.

HLSL circumvents the binary backend:

[RFC] Adding HLSL and DirectX support to Clang & LLVM

Hello all, The HLSL compiler community is interested in contributing HLSL, DirectX and Vulkan graphics support to Clang and LLVM. Why do we want to do this? The existing HLSL compiler, the DirectX Shader Compiler (DXC), is a fork of LLVM/Clang 3.7 and is developed open source on GitHub (https://github.com/microsoft/DirectXShaderCompiler) by Microsoft and a diverse community of open source partners. We plan to update our compiler to the latest LLVM. This plan is motivated by our efforts to bri…