Re-apply "[ORC] LLJIT updates: ExecutorNativePlatform, default ..." with fixes.

This reapplies 371cb1af, which was reverted in 0b2240ed due to bot
failures.

The clang-repl test failure is fixed by dropping the process symbols definition
generator that was manually attached to the main JITDylib, since LLJIT now
exposes process symbols by default. (The bug was triggered when JIT'd code used
the process atexit provided by the generator, rather than the JIT atexit which
has been moved into the platform JITDylib).

Any LLJIT clients that see crashes in static destructors should likewise remove
any process symbol generators attached to their main JITDylib.