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

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cor3ntin wrote:

Can you add a release note indicating https://github.com/llvm/llvm-project/issues/74016 was fixed?
Otherwise LGTM.

If you wanted to make a follow up {R to rename `isPure` to `isPureVirtual`, i think that might be helpful (to avoid confusion with the `gnu::pure` attribute)

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