[5.9] IRGen: alloc_global and global_addr instructions need to agree on the storage

[aschwaighofer]

The ABI used for some typed globals is similar to existential values. We reserve an "inline" 3 word buffer, query type metadata's size, and if the size is bigger allocate storage on the heap.

The global_addr instruction needs to follow this logic: If the storage is opaque we need to project to the underlying buffer.

Risk: Low. The change should only affect opaque types (some types).

Without the change any some typed globals bigger than 3 words might clobber storage in memory immediately after it.

e.g. the following code is broken

public protocol Some {}

 public struct Implementer : Some {
   var w = (0, 1, 2, 3, 4)

   public init() { }
 }

 let g : some Some = Implementer()

Original PR: #66507
rdar://109636344

[aschwaighofer]

@swift-ci test