[clang] [Clang] [CodeGen] Perform derived-to-base conversion on explicit object parameter in lambda (PR #89828)

[via cfe-commits]

Sirraide wrote:

Ok, I’ve added a check for CWG 2881 to sema, but there’s at least 2 things I still want to take a look at:
- [ ] Consider emitting the upcast once in the function prologue instead of every time a variable is accessed.
- [ ] Check for invalid explicit object parameters when the lambda is instantiated rather than when it is called.

https://github.com/llvm/llvm-project/pull/89828