Fix linkage calculation of auto member functions returning lambdas

Fix linkage calculation of auto member functions returning lambdas

As described by Richard in https://groups.google.com/a/isocpp.org/d/msg/std-discussion/S1kmj0wF5-g/fb6agEYoL2IJ

we should allow:

template<typename S>
struct A {

template<typename T> static auto default_lambda() {
  return [](const T&) { return 42; };
}

template<class U = decltype(default_lambda<S>())>
  U func(U u = default_lambda<S>()) { return u; }

};

int run2 = A<double>{}.func()(3.14);

int run3 = A<char>{}.func()('a');

This patch allows the code using the same trickery that was used to allow the code in non-member functions at namespace scope.

Please see http://llvm-reviews.chandlerc.com/D1844 for richard's approval.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192166 91177308-0d34-0410-b5e6-96231b3b80d8