The classic DWARF linker avoids `DIEEntry` for `DW_FORM_ref_addr`
references, using raw `DIEInteger` values with manual offset computation
instead. A stale FIXME explains this was because "the implementation
calls back to DwarfDebug to find the unit offset", but this is no longer
true. `DIEEntry` resolves offsets via
`DIEUnit::getDebugSectionOffset()`, which has no `DwarfDebug`
dependency.
And the real constraint is that forward references may point to
placeholder `DIEs` that never get adopted into a unit tree (due toODR
pruning), so `DIEEntry` cannot resolve them(a test failed during
refactoring this). However, backward references are safe, the target DIE
is already cloned and parented in a unit tree.
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