dillon1000/react

Bugfix: Flush legacy sync passive effects at beginning of event (#21846)

* Re-land recent flushSync changes

Adds back #21776 and #21775, which were removed due to an internal
e2e test failure.

Will attempt to fix in subsequent commits.

* Failing test: Legacy mode sync passive effects

In concurrent roots, if a render is synchronous, we flush its passive
effects synchronously. In legacy roots, we don't do this because all
updates are synchronous — so we need to flush at the beginning of the
next event. This is how `discreteUpdates` worked.

* Flush legacy passive effects at beginning of event

Fixes test added in previous commit.