[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

llvm-svn: 295312
diff --git a/clang/lib/Format/BreakableToken.h b/clang/lib/Format/BreakableToken.h
index d507635..3f83cfe 100644
--- a/clang/lib/Format/BreakableToken.h
+++ b/clang/lib/Format/BreakableToken.h
@@ -358,6 +358,10 @@
 
   // Either "* " if all lines begin with a "*", or empty.
   StringRef Decoration;
+
+  // If this block comment has decorations, this is the column of the start of
+  // the decorations.
+  unsigned DecorationColumn;
 };
 
 class BreakableLineCommentSection : public BreakableComment {