Work around a type checking error when using the Static Linux SDK (#2898

)

# Motivation

Building `swift-nio` with the Static Linux SDK

(https://www.swift.org/documentation/articles/static-linux-getting-started.html),
fails in `PosixSingletons+ConcurrencyTakeOver.swift`:

```
Building for debugging...
error: emit-module command failed with exit code 1 (use -v to see invocation)
/private/tmp/vaportest/.build/checkouts/swift-nio/Sources/NIOPosix/PosixSingletons+ConcurrencyTakeOver.swift:76:12: error: conflicting arguments to generic parameter 'T' ('Optional<UnsafeRawPointer>.AtomicRepresentation' vs. 'Optional<UnsafeRawPointer>.AtomicRepresentation')
 74 |             dlopen(nil, RTLD_NOW),
 75 |             "swift_task_enqueueGlobal_hook"
 76 |         )?.assumingMemoryBound(to: UnsafeRawPointer?.AtomicRepresentation.self)
    |            `- error: conflicting arguments to generic parameter 'T' ('Optional<UnsafeRawPointer>.AtomicRepresentation' vs. 'Optional<UnsafeRawPointer>.AtomicRepresentation')
 77 |         guard let concurrencyEnqueueGlobalHookPtr = concurrencyEnqueueGlobalHookPtr else {
 78 |             return false
```

This happens because `Foundation` and `swift-atomics` both define
`AtomicRepresentation`. A compiler problem (rdar://132885963) makes
these extensions globally visible, causing the two
`AtomicRepresentation`
types to collide.   This only affects non-Darwin platforms.

 # Modification

This PR works around the problem by using a local type alias which does
not collide with the other `AtomicRepresentation` types.

 # Result

It is now possible to build `swift-nio` with the Static Linux SDK.

Fixes: #2893
Suggested-by: Alastair Houghton <ahoughton@apple.com>

Sources/NIOPosix/PosixSingletons+ConcurrencyTakeOver.swift

@@ -73,7 +73,7 @@ extension NIOSingletons {
  let concurrencyEnqueueGlobalHookPtr = dlsym(
  dlopen(nil, RTLD_NOW),
  "swift_task_enqueueGlobal_hook"
- )?.assumingMemoryBound(to: UnsafeRawPointer?.AtomicRepresentation.self)
+ )?.assumingMemoryBound(to: UnsafeRawPointer?.AtomicRep.self)
  guard let concurrencyEnqueueGlobalHookPtr = concurrencyEnqueueGlobalHookPtr else {
  return false
  }
@@ -126,3 +126,12 @@ extension NIOSingletons {
  #endif
  }
 }
+
+// Workaround for https://github.com/apple/swift-nio/issues/2893
+extension Optional
+where
+ Wrapped: AtomicOptionalWrappable,
+ Wrapped.AtomicRepresentation.Value == Wrapped
+{
+ typealias AtomicRep = Wrapped.AtomicOptionalRepresentation
+}