A step to address https://github.com/llvm/llvm-project/issues/62707.
It is not user friendly enough to drop the implicitly generated path
directly. Let's emit the warning first and drop it in the next version.
Close https://github.com/llvm/llvm-project/issues/56916
Within C++20 modules, we may have multiple same constructors in
multiple same RecordDecls. And it doesn't make sense naturally to create
duplicated deduction guides for the duplicated constructors.