[clang] [llvm] [DirectX] Add DirectXTargetCodeGenInfo (PR #104856)

[Damyan Pepper via cfe-commits]

https://github.com/damyanp approved this pull request.

I've a few questions here, but otherwise LGTMy non-expert eyes.  Would be good to get a review from @bogner before completing.

https://github.com/llvm/llvm-project/pull/104856