Add Xcode 11.5

[tgoyne]

This ended up being a bunch of stuff because either installing Xcode 11.5 or updating to macOS 10.15.4 made the first-launch initialization of the simulators' dyld cache a lot slower, which lead to jobs timing out. To deal with this, I made a pile of adjustments:

1. A bunch of jobs were building Realm.framework multiple times. In some cases because it wasn't reusing the one built for tests when building the examples, but in the cocoapods case it also was pointlessly testing the obj-c pod and then the swift pod even though the swift pod test covered everything the obj-c test does.
2. Simulators were being reset a lot more than they needed to be. Resetting them during the first init would start over the dyld cache population, and even when that didn't happen it added a minute or two onto each job. Now simulators are reset at most once per job, and a bunch of jobs which don't actually need a simulator don't touch them at all. The Apple TV jobs now preboot the correct simulator rather than an iPhone simulator that would go unused.
3. Node 8 is out of LTS and seems to have finally stopped working with Xcode 11.5 so I upgraded to node 10.
4. reset-simulators.rb now waits for the dyld cache population to finish. This appears to be faster overall than trying to do it in parallel with the build, and avoids some spurious test failures when it's still running at the same time as tests.
5. Some assorted minor speedups such as not generating debug symbols for things which we just build and then delete immediately without running.

I think all of the remaining failures are pre-existing problems now, such as that one of the Combine tests sometimes deadlocks due to a bug in that test. I'll deal with those separately.

[jsflax]

Looks good. There are files here I haven't looked at before, so I need to take a deeper look at those, but looks generally sound.