DebugInfo: Rebuild dwp debug_info index column from v5 indexes more robustly

the v4 rebuilding is a best-effort because it's not possible to reliably
parse the DWO ID as it requires the abbrev section (& if the index isn't
trustworthy then there's no way to find the associated abbrev section
contribution for a given info section contribution)

But in v5 the DWO ID/type signature is in the header and can be rebuilt
losslessly (only at the cost of performance of rescanning/parsing the
headers of all the units), so let's implement that.

the testing isn't /ideal/ - I think the testing should've been
implemented as a hardcoded dwp file with a corrupted/incorrect index,
then the test could've demonstrated that reparsing the index produces
the right answer - but this is a quick port of the existing v5 test back
to v4 so that we don't lose coverage on the v4 codepath now that it's
separated from the v5 codepath.

Differential Revision: https://reviews.llvm.org/D146662

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