[llvm] [DirectX][NFC] Refine DXIL Type abstraction framework in DXIL.td (PR #81692)

[Chris B via llvm-commits]

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

Parts of this change (like replacing strings with TableGen values) are awesome and I'm all on board.

I question whether we should have a DXIL type abstraction at all. DXIL types are effectively just a subset of valid LLVM types. We could use the LLVM type abstractions that we get from Intrinsics.td to provide consistency between DXIL.td and IntrinsicsDirectX.td.

https://github.com/llvm/llvm-project/pull/81692