clang-format: Support commas in lambda return types.

Before:
  auto next_pair = [](A * a) -> pair<A*, A*>{};

After:
  auto next_pair = [](A* a) -> pair<A*, A*>{};

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@223652 91177308-0d34-0410-b5e6-96231b3b80d8