This change adds support for WASI in stdlib tests. Some tests that expect a crash to happen had to be disabled, since there's currently no way to observe such crash from a WASI host.
This decreases total testing time by over a minute on my old Mac Pro.
It probably has much less effect on systems with fewer cores, but shouldn't
be any worse there.
Swift SVN r22745
Now that it disables access control, this lit definition applies to many
more tests, where we used to use %target-run-stdlib-swift before access
control came online.
Also, drop -Xfrontend -disable-access-control from a bunch of tests that
don't need it.
Swift SVN r22634