Re-apply "[ORC] Unify task dispatch across ExecutionSession..." with more fixes.

This re-applies 6094b3b7, which was reverted in e7efd37c (and before that
in 1effa19d) due to bot failures.

The test failures were fixed by having SelfExecutorProcessControl use an
InPlaceTaskDispatcher by default, rather than a DynamicThreadPoolTaskDispatcher.
This shouldn't be necessary (and indicates a concurrency issue elsewhere), but
InPlaceTaskDispatcher is a less surprising default, and better matches the
existing behavior (compilation on current thread by default), so the change
seems reasonable. I've filed https://github.com/llvm/llvm-project/issues/89870
to investigate the concurrency issue as a follow-up.

Coding my way home: 6.25133S 127.94177W