Within Drake, QDLDL is used by OSQP and SCS.

When changing the release of either QDLDL or OSQP used by Drake, ideally try to
keep Drake's pin of QDLDL aligned with what Drake's pin of OSQP desires.  (SCS
tends to use stale copies of QDLDL and we can't really be expected to stick
with those old versions just for the sake of SCS.)

For example, if Drake pinned OSQP to commit = "v0.4.1" then look at
https://github.com/osqp/osqp/tree/v0.4.1/lin_sys/direct/qdldl
which shows that OSQP 0.4.1 prefers a pin of 7ab0fca for QDLDL.

Looking at https://github.com/osqp/qdldl/commit/7ab0fca we see that it was
approximately the 0.1.3 release.  (The actual v0.1.3 release tag on QDLDL is on
the merge commit of 7ab0fca onto master.)  So, Drake should ideally pin QDLDL
commit = "v0.1.3".

Given the potential interactions of QDLDL, OSQP, and SCS it may not be possible
to keep all of the versions aligned perfectly.  When in doubt, prefer to use
the newest tagged release of all three, patching them as necessary to get
everything working.
