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

[via cfe-commits]

https://github.com/cor3ntin approved this pull request.

I am happy with the lexing/sema changes.
Please give @AaronBallman the opportunity to look at the code gen bits.

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