[clang] [llvm] Add macro to suppress -Wunnecessary-virtual-specifier (PR #139614)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `bolt-x86_64-ubuntu-dylib` running on `bolt-worker` while building `clang,llvm` at step 5 "build-bolt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/119/builds/5689

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build-bolt) failure: build (failure)

```

</details>

https://github.com/llvm/llvm-project/pull/139614