[clang-format] Support namespaces ending in semicolon

Summary:
This patch adds support for namespaces ending in semicolon to the namespace comment fixer.
source:
```
namespace A {
  int i;
  int j;
};
```
clang-format before:
```
namespace A {
  int i;
  int j;
} // namespace A;
```
clang-format after:
```
namespace A {
  int i;
  int j;
}; // namespace A
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D30688

llvm-svn: 297140