[clang-format] Fix regression merging comments across newlines.

Summary:
This fixes a regression that causes example:
```
enum A {
  a, // line a

  // line b
  b
};
```
to be formatted as follows:
```
enum A {
  a, // line a
     // line b
  b
};
```

Reviewers: djasper, klimek

Reviewed By: klimek

Subscribers: cfe-commits, sammccall, djasper, klimek

Differential Revision: https://reviews.llvm.org/D29322

llvm-svn: 293624

1 files changed, 17 insertions, 0 deletions

diff --git a/clang/lib/Format/BreakableToken.cpp b/clang/lib/Format/BreakableToken.cpp
index dd28ba7d4a2..82e62ac80f8 100644
--- a/clang/lib/Format/BreakableToken.cpp
+++ b/clang/lib/Format/BreakableToken.cpp
@@ -681,6 +681,23 @@ BreakableLineCommentSection::BreakableLineCommentSection(
       Content[i] = Content[i].substr(0, EndOfLine);
     }
     LineTok = CurrentTok->Next;
+    if (CurrentTok->Next && CurrentTok->Next->NewlinesBefore > 1) {
+      // A line comment section needs to broken by a line comment that is
+      // preceded by at least two newlines. Note that we put this break here
+      // instead of breaking at a previous stage during parsing, since that
+      // would split the contents of the enum into two unwrapped lines in this
+      // example, which is undesirable:
+      // enum A {
+      //   a, // comment about a
+      //
+      //   // comment about b
+      //   b
+      // };
+      //
+      // FIXME: Consider putting separate line comment sections as children to
+      // the unwrapped line instead.
+      break;
+    }
   }
 }