[clang] [llvm] [HLSL] Implement Texture2D default template (PR #184207)

[Helena Kotas via llvm-commits]

hekota wrote:

This PR contains changes from #183323 which makes it harder to review. Could these two be separated? 

If not, did you know that you can push a user branch to the llvm project (as long as it is named `users/alias/*`), and then create a PR against that branch? That makes it easier to review just the code that is added on top of the first change. After the first one gets merged, the PR can be updated to target `main` and run the verification builds, but it already has a head start on reviews.

See my two latest PRs related to resources in structs for an example. (I don't want to link them here because they are not really related to the Texture2D work.)

https://github.com/llvm/llvm-project/pull/184207