clang-format: Handle comments in short case labels.

With AllowShortCaseLabelsOnASingleLine set to true:
This gets now left unchanged:
  case 1:
    // comment
    return;

Whereas before it was changed into:
  case 1: // comment return;

This fixes llvm.org/PR21630.

llvm-svn: 222529
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index a6bbe16..c58ac0a 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -745,6 +745,11 @@
                "case 3:\n"
                "case 4:\n"
                "case 5: return;\n"
+               "case 6: // comment\n"
+               "  return;\n"
+               "case 7:\n"
+               "  // comment\n"
+               "  return;\n"
                "default: y = 1; break;\n"
                "}",
                Style);