Reland "[clangd] Add tweak to override pure virtuals" (#150788)

This relands commit
https://github.com/llvm/llvm-project/commit/7355ea3f6b214d1569da43d02f9a166ff15012e6.

The original commit was reverted in
https://github.com/llvm/llvm-project/commit/bfd73a5161608e6355f7db87dc5f5afee56d7e2f
because it was breaking the buildbot.

The issue has now been resolved by
https://github.com/llvm/llvm-project/commit/38f82534bbe9e1c9f5edd975a72e07beb7048423.

Original PR: https://github.com/llvm/llvm-project/pull/139348
Original commit message:
<blockquote>

closes https://github.com/clangd/clangd/issues/1037 
closes https://github.com/clangd/clangd/issues/2240

Example:

```c++
class Base {
public:
  virtual void publicMethod() = 0;

protected:
  virtual auto privateMethod() const -> int = 0;
};

// Before:
//                        // cursor here
class Derived : public Base{}^ ;

// After:
class Derived : public Base {
public:
  void publicMethod() override {
    // TODO: Implement this pure virtual method.
    static_assert(false, "Method `publicMethod` is not implemented.");
  }

protected:
  auto privateMethod() const -> int override {
    // TODO: Implement this pure virtual method.
    static_assert(false, "Method `privateMethod` is not implemented.");
  }
};
```


https://github.com/user-attachments/assets/79de40d9-1004-4c2e-8f5c-be1fb074c6de

</blockquote>