[Stdlib] Eagerly realize EmptyDictionarySingleton and EmptySetSingleton.

[mikeash]

These objects can escape into ObjC without their class being realized first, which can cause a crash if the unrealized class gets passed into the ObjC runtime.

rdar://problem/59295395

[mikeash]

@swift-ci please test

[mikeash]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 12ed357aef27991504b2849d2241a3863ef51aa4

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 12ed357aef27991504b2849d2241a3863ef51aa4

[Catfish-Man]

Huh hadn’t seen that attribute to me. Makes sense though!

[mikeash]

Yeah, I hadn't seen it before either. That seemed like the most straightforward fix, so I went hunting for it really hoping it existed. Thankfully, it does.

[DougGregor]

You can thank NSCoding for this funny little attribute, I think

[compnerd]

@DougGregor this reminds me of an issue with lazy value witness table initialization. In Foundation, we have an issue where the value witness for _NSCFConstantString is not realized before the first use. This results in an invalid memory access as the VW is not ready for use. There is a pretty clever/horrible workaround of:

if let _ = "" as? _NSCFConstantString { }

since we do not have a convenient way to invoke swift_getForeignTypeMetadata.

Thoughts on having something similar for that case?