[lldb] From unordered_map synthetic provider, return std::pair children

Change the behavior of the libc++ `unordered_map` synthetic provider to present
children as `std::pair` values, just like `std::map` does.

The synthetic provider for libc++ `std::unordered_map` has returned children
that expose a level of internal structure (over top of the key/value pair). For
example, given an unordered map initialized with `{{1,2}, {3, 4}}`, the output
is:

```
(std::unordered_map<int, int, std::hash<int>, std::equal_to<int>, std::allocator<std::pair<const int, int> > >) map = size=2 {
  [0] = {
    __cc = (first = 3, second = 4)
  }
  [1] = {
    __cc = (first = 1, second = 2)
  }
}
```

It's not ideal/necessary to have the numbered children embdedded in the `__cc`
field.

Note: the numbered children have type
`std::__hash_node<std::__hash_value_type<Key, T>, void *>::__node_value_type`,
and the `__cc` fields have type `std::__hash_value_type<Key, T>::value_type`.

Compare this output to `std::map`:

```
(std::map<int, int, std::less<int>, std::allocator<std::pair<const int, int> > >) map = size=2 {
  [0] = (first = 1, second = 2)
  [1] = (first = 3, second = 4)
```

Where the numbered children have type `std::pair<const Key, T>`.

This changes the behavior of the synthetic provider for `unordered_map` to also
present children as `pairs`, just like `std::map`.

It appears the synthetic provider implementation for `unordered_map` was meant
to provide this behavior, but was maybe incomplete (see
d22a9437). It has both an `m_node_type` and an
`m_element_type`, but uses only the former. The latter is exactly the type
needed for the children pairs. With this existing code, it's not much of a
change to make this work.

Differential Revision: https://reviews.llvm.org/D117383