Update Eigen to commit:b396a6fbb2e173f52edb3360485dedf3389ef830

CHANGELOG
=========
b396a6fbb - Add free-function swap.
820e8a45f - add compile time info to reverse in place
b55dab7f2 - Fix DenseBase::tail for Dynamic template argument
e0cbc55d9 - Update README.md

PiperOrigin-RevId: 686151157
Change-Id: Ib5c5a685c6277a21aa6fd75f5468de4c42461ddd
6 files changed