[LLDB] Fix MS STL `variant` with non-trivial types (#171489)
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`).