The general policy has been that even if something crashes at run
time, we don't make it a hard error in Swift 4 mode (or Swift 3 mode!)
if it wasn't a hard error in Swift 4.0 (3.0). In this case, we thought
we could get away with it, and then it turns out it actually caused
some problems. (And as 2bc010681 shows, we can still make mistakes.)
This change isn't perfect because the diagnostic appears in /clients/
rather than in the module that's being compiled as Swift 4 (instead of
Swift 5). But it still means that someone who hasn't changed
/anything/ from a valid Swift 4.0 project will be able to compile
without any changes, even if they were relying on being able to call
+new when -init was unavailable for some reason.
More rdar://problem/35942058
Currently a loop-hole exists by which one could end up invoking an unavailable -init initializer from Obj-C
when it is unavailable by using +new (which itself calls -init)
https://bugs.swift.org/browse/SR-5018
Now that Clang has availability diagnostics too
(-Wpartial-availability and friends) we need to get this correct, or
people will get warnings in the generated header!
rdar://problem/33313703
