[LLDB] Add formatters for MSVC STL std::atomic (#149801)
Adds synthetic children and a summary provider for `std::atomic` on
MSVC's STL. This currently only supports DWARF because it relies on the
template argument. Once there are PDB tests, this will probably use the
return type of some method like `value()` because template types aren't
available there.
Towards #24834.
