Migrate DirectX backend to LLVM

[alexreinking]

Problem statement

The existing DirectX backend was written as an HLSL source-based generator. This architecture suffers from a variety of issues, basically all the reasons we go through LLVM's NVPTX backend rather than generating CUDA sources.

Proposed solution

LLVM has an (experimental) DirectX backend that supports SM6.0+ (i.e. only DirectX 12+). We should investigate using this and the viability of deprecating support for DirectX 11.

https://llvm.org/docs/DirectXUsage.html

Additional context

LLVM can be built with DirectX support by setting LLVM_EXPERIMENTAL_TARGETS_TO_BUILD=DirectX.