[clang-tidy] Handle unique owning smart pointers in ExprMutationAnalyzer

Summary:
For smart pointers like std::unique_ptr which uniquely owns the
underlying object, treat the mutation of the pointee as mutation of the
smart pointer itself.

This gives better behavior for cases like this:
```
void f(std::vector<std::unique_ptr<Foo>> v) { // undesirable analyze result of `v` as not mutated.
  for (auto& p : v) {
      p->mutate(); // only const member function `operator->` is invoked on `p`
  }
}
```

Reviewers: hokein, george.karpenkov

Subscribers: xazax.hun, a.sidorin, Szelethus, cfe-commits

Differential Revision: https://reviews.llvm.org/D50883

llvm-svn: 341967