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

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

Can you 
 - use `DiagCompat`
 - Add a test to show we are not accidentally pulling-in concepts through type constraints
 - update LanguageExtension.rst
 - Add a release note.


Thanks

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