Data formatter support for libc++ std::atomic<T>

On libc++ std::atomic is a fairly simple data type (layout wise, at least), wrapping actual contents in a member variable named "__a_"

All the formatters are doing is "peel away" this intermediate layer and exposing user data as direct children or values of the std::atomic root variable

Fixes rdar://24329405

llvm-svn: 260752