[dsymutil] Track uncloned references in the DIEInfo.

To improve deduplication across CUs within a single object file,
2b747241 changed the way we track ODR canonical candidates. The
result is that some assumptions no longer hold. Because of the
aforementioned change, the following condition

  assert(Ref > InputDIE.getOffset());

in DWARFLinker::DIECloner::cloneDieReferenceAttribute is no longer an
invariant. An example of a situation where this assertion no longer
holds is when we have decided to replace a backward reference in the
current CU with a forward reference in a subsequent CU. The ODR
canonical DIE hasn't been emitted yet, but the references DIE has an
offset smaller than the current DIE. The assertion is only true if the
referenced DIE was the ODR canonical DIE, which is no longer guaranteed
to be part of the same CU.

Big thanks to Alexey for putting a test together.

Differential revision: https://reviews.llvm.org/D138176