Update Eigen to commit:b6849f675d4237398c4a3ae806ddaf4ca461ed01

CHANGELOG
=========
b6849f675 - Change the midpoint chosen in Eigen::ForkJoinScheduler.
1b2e84e55 - Fix minor typos in `ForkJoin.h`
872c179f5 - Fix UTF-8 encoding errors introduced by #1801

PiperOrigin-RevId: 721553862
Change-Id: I44149f69a94bfdc8b3df291521bdf3effc69878e
2 files changed