[clang-tidy] readability-redundant-declaration false positive for defaulted function

Summary:
```
template <class T>
struct C {
  C();
};

template <class T>
C<T>::C() = default;
```

Causes a readability-redundant-declaration diagnostic. This is caused by `isDefinition` not matching defaulted functions.

Reviewers: alexfh, danielmarjamaki

Reviewed By: alexfh

Subscribers: xazax.hun, cfe-commits

Patch by Florian Gross!
Differential Revision: https://reviews.llvm.org/D33358

llvm-svn: 303552