[clang] [NFC][Clang][CodeGen] Improve performance for vtable metadata generation (PR #67066)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

Please next time before you commit add a more detailed description of the change so that readers of git log can get a better understanding of the change w/o having to view it in detail.

https://github.com/llvm/llvm-project/pull/67066