[Sema] Maintain ellipsis location when transforming lambda captures

This patch fixes a crash caused by the following case:

  template<typename T>
  auto f(T x) {
    auto g = [](auto ... args) {
      auto h = [args...]() -> int {
        return 0;
      };
      return h;
    };
    return g;
  }

  auto x = f(0)();

When the templated function 'f' is instantiated and the inner-most
lambda is transformed the ellipsis location on the captured variable
is lost.  Then the lambda returned by 'f' is instantiated and the
tree transformer chokes on the invalid ellipsis location.  The
problem is fixed by making a minor change to properly track the
ellipsis location.

This fixes PR23716.

Differential Revision: http://reviews.llvm.org/D10590

llvm-svn: 240740