When using an experimental feature that is not available
in production, a test needs to have `REQUIRES: asserts`
so that non-assert builds don't unexpectedly fail.
The latter example NIODefaultSerialEventLoopExecutor when the type is
more available than the enqueue implementation provided via an
extrension would issue a warning that that this enqueue(ExecutorJob)
would not be used which is not true.
We need to filter out all "default impls" from the stdlib as we issue
this warning.
We also put a note on the offending declaration that one can remove now,
to ease getting rid of warnings when possible
Resolves rdar://115166475
