[clang] [Clang] Follow up to previous lambda-in-concept patch (PR #210381)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

It isn't clear tome how exactly we got here, can you give some background/explain how/why this patch is working?  How were we getting negative numbers in the first place?

@mizvekov : Would definitely like your eyes on this, as you've messed with the templ arg lists a bunch lately.

https://github.com/llvm/llvm-project/pull/210381