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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

@kstoimenov I believe you are supposed to notify the MR in case of reverts, otherwise I could have missed this if I wasn't awake.

I believe in this case the bot is misconfigured, it's using `-Werror,-Wmissing-field-initializers`, the 'error' is just a missing initializer, which I didn't add because I think it makes the intent clearer.

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