[clang-format] Add a NamespaceEndCommentsFixer

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@296341 91177308-0d34-0410-b5e6-96231b3b80d8