Fix ICE on const eval of union field

MIR's `Const::get_field()` attempts to retrieve the value for a given
field in a constant. In the case of a union constant it was falling
through to a generic `const_get_elt` based on the field index. As union
fields don't have an index this caused an ICE in `llvm_field_index`.

Fix by simply returning the current value when accessing any field in a
union. This works because all union fields start at byte offset 0.

The added test uses `const_fn` it ensure the field is extracted using
MIR's const evaluation. The crash is reproducible without it, however.

Fixes #47788

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