[clang] [HLSL] Add `[[hlsl::contained_type()]]` attribute (PR #108456)

[Helena Kotas via cfe-commits]

hekota wrote:

> Why the name is contained_type instead of element_type?

I am not sure if the name was discussed but `contained_type` seems a bit more general than `element_type`. We can always change it as we iron out details of other resources type like textures or samplers, and change it if the name is no longer a good fit.

https://github.com/llvm/llvm-project/pull/108456