Add IWYU private pragmas to internal headers.
Generated via recurseive sed based on looking for "InternalHeaderCheck.h":
$ find Eigen -type f -exec sed -E -i 's|(#include.*InternalHeaderCheck.h")|// IWYU pragma: private\n\1|' {} +
$ find unsupported -type f -exec sed -E -i 's|(#include.*InternalHeaderCheck.h")|// IWYU pragma: private\n\1|' {} +
This is to help tooling better determine which headers to suggest in auto-completion.