[clangd] Handle destructors in DefineOutline tweak

Fix destructors being incorrectly defined in the DefineOutline tweak
Currently it doesn't prepend the class name to the destructor
```lang=c++
class A { ~A() {} };
// Destructor definition after outline
~A() {}
// After this fix
A::~A() {}
```

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D147802