Change unsafeDownCast to as! (#2802)

# Motivation

In Swift 5.10 the usage of `unsafeDownCast` can lead to a miss-compile which will result in bad runtime behaviour.

# Modification

This PR changes the `unsafeDownCast` to use a `as!` instead. This is safe and should result in the same performance when done with `ManagedBuffer` which is inlinable.

# Result

No more miss compiles in 5.10

Sources/NIOConcurrencyHelpers/NIOLock.swift

@@ -130,7 +130,10 @@ final class LockStorage<Value>: ManagedBuffer<Value, LockPrimitive> {
         let buffer = Self.create(minimumCapacity: 1) { _ in
             value
         }
-        let storage = unsafeDowncast(buffer, to: Self.self)
+        // Intentionally using a force cast here to avoid a miss compiliation in 5.10.
+        // This is as fast as an unsafeDownCast since ManagedBuffer is inlined and the optimizer
+        // can eliminate the upcast/downcast pair
+        let storage = buffer as! Self
 
         storage.withUnsafeMutablePointers { _, lockPtr in
             LockOperations.create(lockPtr)

Sources/NIOPosix/Pool.swift

@@ -164,7 +164,11 @@ extension PooledBuffer {
             }
 
             // Here we set up our memory bindings.
-            let storage = unsafeDowncast(baseStorage, to: Self.self)
+
+            // Intentionally using a force cast here to avoid a miss compiliation in 5.10.
+            // This is as fast as an unsafeDownCast since ManagedBuffer is inlined and the optimizer
+            // can eliminate the upcast/downcast pair
+            let storage = baseStorage as! Self
             storage.withUnsafeMutablePointers { headPointer, tailPointer in
                 UnsafeRawPointer(tailPointer + headPointer.pointee.iovectorOffset).bindMemory(
                     to: IOVector.self,
@@ -277,7 +281,10 @@ struct PooledMsgBuffer: PoolElement {
                 head
             }
 
-            let storage = unsafeDowncast(baseStorage, to: Self.self)
+            // Intentionally using a force cast here to avoid a miss compiliation in 5.10.
+            // This is as fast as an unsafeDownCast since ManagedBuffer is inlined and the optimizer
+            // can eliminate the upcast/downcast pair
+            let storage = baseStorage as! Self
             storage.withUnsafeMutablePointers { headPointer, tailPointer in
                 UnsafeRawPointer(tailPointer + headPointer.pointee.msgHdrsOffset).bindMemory(
                     to: MMsgHdr.self,