[clang] [llvm] [DirectX] Implement UseNativeLowPrecision shader flag analysis (PR #134288)

[Finn Plummer via cfe-commits]

https://github.com/inbelic commented:

Assuming `-fnative-half-type` can be directly used as an alias to `enable-16bit-types` in DXC, this LGTM. Just some nits otherwise

https://github.com/llvm/llvm-project/pull/134288