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/lib/Format/Format.cpp b/clang/lib/Format/Format.cpp
index 729ca97..b96c17a 100644
--- a/clang/lib/Format/Format.cpp
+++ b/clang/lib/Format/Format.cpp
@@ -750,7 +750,7 @@
       if (Line->First->isOneOf(tok::kw_case, tok::kw_default, tok::r_brace))
         break;
       if (Line->First->isOneOf(tok::kw_if, tok::kw_for, tok::kw_switch,
-                               tok::kw_while))
+                               tok::kw_while, tok::comment))
         return 0;
       Length += I[1 + NumStmts]->Last->TotalLength + 1; // 1 for the space.
     }