[clang] [Clang](NFC) Add coverage for VTable debug info (PR #151818)

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

I'd like to add test coverage and verify the intended behavior on edge cases before submitting the long-term fix for #149639.

@jmorse @CarlosAlbertoEnciso  
Could you take a look at this and check if the new tests reflect the intended behavior?

https://github.com/llvm/llvm-project/pull/151818