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

[via cfe-commits]

cor3ntin wrote:

This is the documentation i found.
Can you confirm the intent is only to support `.x???` and `r???` ?
It alludes to more options.
Maybe we need a `isHLSLSwizzleStart` function to avoid comparing to `'x'` and `'r'` in multiple places.

https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx9-graphics-reference-asm-vs-registers-modifiers-source-swizzling
https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx9-graphics-reference-asm-ps-registers-modifiers-source-register-swizzling  

I don't suppose there is a specification / grammar for these things?

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