[clang] [clang] Disable missing definition warning on pure virtual functions (PR #74510)

[Charalampos Mitrodimas via cfe-commits]

charmitro wrote:

> LGTM! Do you need someone to land this on your behalf?

I don't have commit rights. If I remember correctly from when LLVM used Phabricator (reviews.llvm.org) you'd need my name & e-address. If that's so,
`Charalampos Mitrodimas <charmitro at posteo.net>`

https://github.com/llvm/llvm-project/pull/74510