[clang-tidy] cppcoreguidelines-virtual-class-destructor should ignore final classes

The `cppcoreguidelines-virtual-class-destructor` supposed to enforce
http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c35-a-base-class-destructor-should-be-either-public-and-virtual-or-protected-and-non-virtual

Quote:
> A **base** class destructor should be either public and virtual, or
> protected and non-virtual
[emphasis mine]

However, this check still rules the following case:

  class MostDerived final : public Base {
  public:
    MostDerived() = default;
    ~MostDerived() = default;
    void func() final;
  };

Even though `MostDerived` class is marked `final`, thus it should not be
considered as a **base** class. Consequently, the rule is satisfied, yet
the check still flags this code.

In this patch, I'm proposing to ignore `final` classes since they cannot
be //base// classes.

Reviewed By: whisperity

Differential Revision:  https://reviews.llvm.org/D126891