[clang] [HLSL] Support vector swizzles on scalars (PR #67700)

[Chris B via cfe-commits]

llvm-beanz wrote:

@cor3ntin, I've in parallel been working on fleshing out the immediately relevant bits of our language spec. This PR describes the _pp-number_ and _vector-literal_ grammars roughly correctly to this change.

I think I've addressed all the other feedback on the PR, please let me know if there's anything else you think I should change.

https://github.com/llvm/llvm-project/pull/67700