The layouts of resilient value types shipped in the Swift 5 standard library
x and overlays will forever be frozen in time for backward deployment to old
Objective-C runtimes. This PR ensures that even if the layouts of these types
evolve in the future, binaries built to run on the old runtime will continue
to lay out class instances in a manner compatible with Swift 5.
Fixes <rdar://problem/45646886>.
This ensures that the trivial property descriptors is externally
accessible on targets like Windows which does not export symbols by
default. Thanks to Joe Groff for the pointer about the global aliases
for trivial property descriptors!
