[PATCH] [libcxx] Delay evaluation of __make_tuple_types to prevent blowing the max template instantiation depth. Fixes Bug #18345

[Eric Fiselier]

Hi mclow.lists,

http://llvm.org/bugs/show_bug.cgi?id=18345

Tuple's constructor and assignment operators for "tuple-like" types evaluates __make_tuple_types unnecessarily. In the case of a large array this can blow the template instantiation depth.

Ex:
```
#include <array>
#include <tuple>
#include <memory>
 
typedef std::array<int, 1256> array_t;
typedef std::tuple<array_t> tuple_t;

int main() {
  array_t a;
  tuple_t t(a); // broken
  t = a; // broken

  // make_shared uses tuple behind the scenes. This bug breaks this code.
  std::make_shared<array_t>(a);
}
```

To prevent this from happening we delay the instantiation of `__make_tuple_types` until after we perform the length check. Currently `__make_tuple_types` is instantiated at the same time that the length check .

http://reviews.llvm.org/D4467

Files:
  include/__tuple
  test/utilities/tuple/tuple.tuple/tuple.assign/tuple_array_template_depth.pass.cpp
  test/utilities/tuple/tuple.tuple/tuple.cnstr/tuple_array_template_depth.pass.cpp
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