[5.10] [stdlib] Avoid materializing strong references in potential dangling unmanaged opaque functions

[Azoy]

• Explanation: Previously, these functions bit casted pointers to a temporary Instance local (and vice versa) and these causes the compiler to emit retain/releases in certain configurations (notably debug). If the pointer we were converting was a dangling reference to an already released class ref for instance, then this would attempt to retain a bogus pointer which caused crashes. Operate on Unmanaged<Instance> going to and from pointers instead to elide these retain/releases that the compiler wants to add.
• Scope: Affects users of Unmanaged.toOpaque and Unmanaged.fromOpaque who should mostly be folks like the standard library.
• Risk: Low. This does not change any existing code relying on the previous semantics, this only fixes previously broken uses of these functions.
• Testing: Added a new test to exercise that this prevents a crash.
• Main branch PR: [stdlib] Avoid materializing strong references in potential dangling unmanaged opaque functions #70945
• Reviewed by: @lorentey @glessard @stephentyrone

[Azoy]

@swift-ci please test