[lldb] Don't model std::atomic as a transparent data structure in the data formatter

Summary:
Currently the data formatter is treating `std::atomic` variables as transparent wrappers
around their underlying value type. This causes that when printing `std::atomic<A *>`, the data
formatter will forward all requests for the children of the atomic variable to the `A *` pointer type
which will then return the respective members of `A`. If `A` in turn has a member that contains
the original atomic variable, this causes LLDB to infinitely recurse when printing an object with
such a `std::atomic` pointer member.

We could implement a workaround similar to whatever we do for pointer values but this patch
just implements the `std::atomic` formatter in the same way as we already implement other
formatters (e.g. smart pointers or `std::optional`) that just model the contents of the  as a child
"Value". This way LLDB knows when it actually prints a pointer and can just use its normal
workaround if "Value" is a recursive pointer.

Fixes rdar://59189235

Reviewers: JDevlieghere, jingham, shafik

Reviewed By: shafik

Subscribers: shafik, christof, jfb, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74310