[clang] [clang] Implement `__is_virtual_base_of()` intrinsic (PR #100393)

[via cfe-commits]

cor3ntin wrote:

The CI issues are unrelated so I'll merge this (someone should investigate why `clang-doc` tests are still failing though @PeterChou1. Thanks)

@Endilll Thanks for the patch :)

https://github.com/llvm/llvm-project/pull/100393