[clang] Fix for incomplete buffer types in HLSL (PR #163823)

[Chris B via cfe-commits]

https://github.com/llvm-beanz requested changes to this pull request.

Calling complete type before the type is completed breaks _a lot_, we also deliberately don't populate or complete the decls until they are used in a context that requires them to be complete. Lazy initializing the decls is a huge performance win.

https://github.com/llvm/llvm-project/pull/163823