[upstream_utils] Update to latest Eigen master

[calcmogul]

There hasn't been a release in 2.5 years.

[sciencewhiz]

This feels like something we'd want more runtime on, since Eigen master probably doesn't have nearly as much as the release from other users. Is there something in master that we need?

[calcmogul]

There's performance improvements for some NEON instructions, UB fixes, a lot of internal cleanup with the jump from C++11 to C++14, and more constexpr. They also ran clang-format on the whole repo right before this commit, so I want the Eigen constexpr patches I'm working on to apply after that.

As for testing, Eigen has a CI test suite for aarch64, PPC, and x86-64 that runs for 6-7 hours (the tests eat a lot of RAM when compiling, so they take a while to build). This CI run from a few days ago is the most recent: https://gitlab.com/libeigen/eigen/-/pipelines/1090190828. The main thing a release goes through that master doesn't is the devs poke at a release candidate first.

Eigen hasn't put out a new release, even for bugfixes, in 2.5 years because their maintainers have a combined 15 minutes per week to work on Eigen (i.e., they don't have the time to maintain the project properly). I've considered migrating us to a better maintained linear algebra library that targets a newer C++ standard, but they all have Fortran library dependencies that are difficult to build and manage. Eigen is the only one with "batteries included" and optional BLAS/LAPACK support.