[clang] [Clang] Disallow explicit object parameters irrespective of whether return type is valid (PR #174603)

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Sirraide wrote:

> > Can you add the reproducer from the issue as a test case as well? Otherwise, this lgtm.
> 
> Thanks for the quick review! The testcase in that issue only works when `-std=c++03` or lower is passed and IMHO, isn't a good representation of the actual issue at hand. Do you still want me to add it?

Yes, it should be added regardless to demonstrate that this patch actually fixes that issue; if it requires C++98 you’ll also have to put it in a separate file

https://github.com/llvm/llvm-project/pull/174603