[clang] [HLSL][Matrix] Implement dynamic single subscript swizzle (PR #173201)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

> Mostly LGTM once we have an issue to track the data-race

[#173201](https://github.com/llvm/llvm-project/issues/174629)

https://github.com/llvm/llvm-project/pull/173201