Reapplying #20254

PR #20430 contained 151 commits, one of which backleveled PR #20254.

This PR is to re-apply that change. Text of 20254:
PR #20229 included a change to the Eigen version that failed to
compile on ppc64le. rmlarsen created a pull request in Eigen to
fix the compile failure:
https://bitbucket.org/eigen/eigen/pull-requests/410

This patch is to pick up the Eigen version of that patch.