[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
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 7348fa0..fb77006 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -934,6 +934,24 @@
                "  VAL_B\n"
                "};");
 
+  EXPECT_EQ("enum A {\n"
+            "  // line a\n"
+            "  a,\n"
+            "  b, // line b\n"
+            "\n"
+            "  // line c\n"
+            "  c\n"
+            "};",
+            format("enum A {\n"
+                   "  // line a\n"
+                   "  a,\n"
+                   "  b, // line b\n"
+                   "\n"
+                   "  // line c\n"
+                   "  c\n"
+                   "};",
+                   getLLVMStyleWithColumns(20)));
+
   verifyFormat(
       "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa =\n"
       "    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb; // Trailing comment");