clang-format: Support lambdas with namespace-qualified return types.

E.g.:
  Foo([]()->std::vector<int> { return { 2 }; }());

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201139 91177308-0d34-0410-b5e6-96231b3b80d8