[clang] [Clang] prevent null explicit object argument from being deduced (PR #104328)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

I'd have expected we would have a previous step analogous to CheckTemplateArgument which would have diagnosed null explicit arguments, before we started trying to deduce anything.

https://github.com/llvm/llvm-project/pull/104328