Add `__reference_binds_to_temporary` trait for checking safe reference initialization.

Summary:
The STL types `std::pair` and `std::tuple` can both store reference types. However their constructors cannot adequately check if the initialization of reference types is safe.  For example:

```
std::tuple<std::tuple<int> const&> t = 42;
// The stored reference is already dangling.
```

Libc++ has a best effort attempts in tuple to diagnose this, but they're not able to handle all valid cases (If I'm not mistaken). For example initialization of a reference from the result of a class's conversion operator.  Libc++ would benefit from having a builtin traits which can provide a much better implementation.

This patch introduce the `__reference_binds_to_temporary(T, U)` trait  that determines whether a reference of type `T` bound to an expression of type `U` would bind to a materialized temporary object.

Note that the trait simply returns false if `T` is not a reference type instead of reporting it as an error.

```
static_assert(__is_constructible(int const&, long));
static_assert(__reference_binds_to_temporary(int const&, long));
```


Reviewers: majnemer, rsmith

Reviewed By: rsmith

Subscribers: compnerd, cfe-commits

Differential Revision: https://reviews.llvm.org/D29930

llvm-svn: 322334