Update Eigen to commit:2873916f1ca24e3282bf6e0150545d34a16b5224

CHANGELOG
=========
2873916f1 - Rename plugin headers to .inc.
6e4d5d483 - Add IWYU private pragmas to internal headers.
328b5f908 - Add temporary macro to allow unaligned scalar UB.
a798d0765 - Fix tensor stridedlinearbuffercopy
8d9f46703 - fix boost mp test to refer to new svd tests
0ae7d7a36 - Fix unaligned scalar alignment UB.

PiperOrigin-RevId: 559221879
Change-Id: I74170dea6585d56ecc6537c52221f1545748c231