[clang-format] Add a NamespaceEndCommentsFixer

Summary:
This patch adds a NamespaceEndCommentsFixer TokenAnalyzer for clang-format,
which fixes end namespace comments.
It currently supports inserting and updating existing wrong comments.

Example source:
```
namespace A {
int i;
}

namespace B {
int j;
} // namespace A
```

after formatting:
```
namespace A {
int i;
} // namespace A

namespace B {
int j;
} // namespace B
```

Reviewers: klimek, djasper

Reviewed By: djasper

Subscribers: klimek, mgorny

Differential Revision: https://reviews.llvm.org/D30269

llvm-svn: 296341