clang-format: Enable formatting of lambdas with explicit return type.

So clang-format can now format:

  int c = []()->int { return 2; }();
  int c = []()->vector<int> { return { 2 }; }();

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@199368 91177308-0d34-0410-b5e6-96231b3b80d8