Because PDB doesn't know about templates, we need to get to `T` of
`std::atomic<T>` differently. The type includes the `value_type`
typedef, which is always equal to `T`. The native PDB plugin includes
this since #169248.
Then we can run the `std::atomic` test with (native) PDB.
From
https://github.com/llvm/llvm-project/pull/163077#issuecomment-3396435083:
Currently, `std::atomic<T>` will always use the MSVC STL synthetic
children and summary. When inspecting types from other STLs, the output
would not show any children.
This PR adds a check that `std::atomic` contains `_Storage` to be
classified as coming from MSVC's STL.
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.
