[clang] [Clang] Accept auto parameters pre-C++20 as an extension (PR #200670)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

>     * Add a test to show we are not accidentally pulling-in concepts through type constraints

Could you share what you have in mind here? for concepts you first kinda need the keywords, which we don't have pre-C++20.

https://github.com/llvm/llvm-project/pull/200670