[libc++] Fix type confusion in hash_{,multi}map
The type `__gnu_cxx::hash_{,multi}map` creates objects of type
`std::pair<Key, Value>` and returns pointers to them of type
`std::pair<const Key, Value>`. If either `Key` or `Value` are
non-standard-layout, this is UB, and is furthermore considered by
pointer field protection to be a type confusion, which leads to a
program crash. Fix it by using the correct type for the pair's storage
and using const_cast to form a pointer to the key in the one place where
that is needed.
Reviewers: ldionne
Reviewed By: ldionne
Pull Request: https://github.com/llvm/llvm-project/pull/183223
