[clang] [HLSL][Matrix] Update indexed matrix elements individually (PR #176216)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

mostly looks good. have a  minor concern that we are blending the HLSL behavior in a way that might make it hard to distinguish in the future if other language modes want to adopt the single subscripting.  I think helpers with hlsl prefixed name or a named lambda might help  if there is a future code reorganization. 

https://github.com/llvm/llvm-project/pull/176216