[cleanup] Remove unused IrField::getDefaultInit.
[cleanup] Remove unused IrTypeAggr::default_fields and accessors.
[cleanup] Merge DtoIndex{Struct, Class}.
Track member field index in IrTypeAggr, not IrField.

This fixes a regression introduced in caa2f15c8a. The IrDsymbol
metadata is obviously reset in between modules, while IrTypes
are not. Thus, we can never set symbol metadata when resolving
types, as the symbol data will be wrong in all modules following
the one where the type was first resolved.

GitHub: Fixes #739.
Merge pull request #740 from klickverbot/field-index

Track member field index in IrTypeAggr, not IrField.