When using `std::variant` with non-trivial types, we need to go through
multiple bases to find the `_Which` member. The MSVC STL implements this
in `xsmf_control.h` which conditionally adds/deletes copy/move
constructors/operators.
We now go to `_Variant_base` (the holder of `_Which`). This inherits
from `_Variant_storage`, which is our entry point to finding the n-th
storage (going through `_Tail`).
Split off from #171489. This only adds the lookup of the active type for
a `std::variant` based on the head type (since PDB doesn't have template
info).
Adds a summary and synthetic children for MSVC STL's `std::variant`.
This one is a bit complicated because of DWARF vs PDB differences. I put
the representations in comments. Being able to `GetChildMemberWithName`
a member in an anonymous union would make this a lot simpler
(`std::optional` will have something similar iirc).
Towards #24834.
