[clang] [clang] Reject 'auto' storage class with type specifier in C++ (PR #166004)

[Tony Guillot via cfe-commits]

https://github.com/to268 commented:

I would like to move some checks as much as possible from the Parsing stage to the Sema stage primarily for maintainability reasons, as we probably have code that parses templates and concept constraints already, and it would be easier to acquire the "parts" you need.

I also would like to see this patch rebased on `main` due to the following commits:
- https://github.com/llvm/llvm-project/commit/211b88b64b0e409a2e294a1fe444b305aa94a2a6
- https://github.com/llvm/llvm-project/commit/42b6a6faaa0014f7e39c57c6221fec1855ec4409

https://github.com/llvm/llvm-project/pull/166004