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

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hax0kartik 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?

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