[llvm] [DirectX][NFC] Change usage pattern DXIL* to Dxil* in DXIL Emitter (PR #80778)

[Chris B via llvm-commits]

https://github.com/llvm-beanz commented:

I have a preference for `DXIL` rather than `Dxil`. My reasoning is that DXIL is an initialism for DirectX Intermediate Language.

There are some places where LLVM's convention recommend it be `dxil` (i.e. `namespace dxil`), which makes sense to me, but I have an aversion to `Dxil`.

https://github.com/llvm/llvm-project/pull/80778