[clang] [Clang] Bugfixes and improved support for `AttributedType`s in lambdas (PR #85325)

[via cfe-commits]

Sirraide wrote:

> so splitting this up would entail adding code to the latter, only to then remove it again in a follow-up patch and move it to a separate function

Though actually, I suppose it would be possible do add `adjustType()` *first* and make the rest of this pr depend on that.

https://github.com/llvm/llvm-project/pull/85325