Support formatting of preprocessor branches.

We now correctly format:
  void SomeFunction(int param1,
  #ifdef X
                    NoTemplate param2,
  #else
                    template <
  #ifdef A
                        MyType<Some> >
  #else
                        Type1, Type2>
  #endif
                    param2,
  #endif
                    param3) {
    f();
  }

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192503 91177308-0d34-0410-b5e6-96231b3b80d8