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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

> It gives a generic `unknown type name 'Foo'`. Do you think that's worth adding somewhere?

Yup. Thanks!

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