[clang-format] Align block comment decorations

Summary:
This patch implements block comment decoration alignment.

source:
```
/* line 1
* line 2
*/
```

result before:
```
/* line 1
* line 2
*/
```

result after:
```
/* line 1
 * line 2
 */
```

Reviewers: djasper, bkramer, klimek

Reviewed By: klimek

Subscribers: mprobst, cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29943

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@295312 91177308-0d34-0410-b5e6-96231b3b80d8