[lldb][Formatters] Consistently unwrap pointer element_type in std::shared_ptr formatters (#147340)

Follow-up to
https://github.com/llvm/llvm-project/pull/147165#pullrequestreview-2992585513

Currently when we explicitly dereference a std::shared_ptr, both the
libstdc++ and libc++ formatters will cast the type of the synthetic
pointer child to whatever the `std::shared_ptr::element_type` is aliased
to. E.g.,
```
(lldb) v p
(std::shared_ptr<int>) p = 10 strong=1 weak=0 {
  pointer = 0x000000010016c6a0
}
(lldb) v *p
(int) *p = 10
```

However, when we print (or dereference) `p.pointer`, the type devolves
to something less user-friendly:
```
(lldb) v p.pointer
(std::shared_ptr<int>::element_type *) p.pointer = 0x000000010016c6a0
(lldb) v *p.pointer
(std::shared_ptr<int>::element_type) *p.pointer = 10
```

This patch changes both formatters to store the casted type. Then
`GetChildAtIndex` will consistently use the unwrapped type.