stdlib: Use builtin tail-allocated arrays instead of ManagedBuffer to allocate contiguous array buffers.

This reduces the amount of SIL generated for array operations significantly.
The generated code should be mostly the same (modulo different inlining decisions).

stdlib/public/core/Arrays.swift.gyb

@@ -950,12 +950,9 @@ func _allocateUninitializedArray<Element>(_  builtinCount: Builtin.Word)
   if count > 0 {
     // Doing the actual buffer allocation outside of the array.uninitialized
     // semantics function enables stack propagation of the buffer.
-    let bufferObject = ManagedBufferPointer<_ArrayBody, Element>(
-      _uncheckedBufferClass: _ContiguousArrayStorage<Element>.self,
-      minimumCapacity: count)
+    let bufferObject = Builtin.allocWithTailElems_1(_ContiguousArrayStorage<Element>.self, builtinCount, Element.self)
 
-    let (array, ptr) = Array<Element>._adoptStorage(
-      bufferObject.buffer, count: count)
+    let (array, ptr) = Array<Element>._adoptStorage(bufferObject, count: count)
     return (array, ptr._rawValue)
   }
   // For an empty array no buffer allocation is needed.
@@ -1106,16 +1103,12 @@ extension ${Self} : RangeReplaceableCollection, _ArrayProtocol {
   @_versioned
   @_semantics("array.uninitialized")
   internal static func _adoptStorage(
-    _ storage: AnyObject, count: Int
+    _ storage: _ContiguousArrayStorage<Element>, count: Int
   ) -> (Array, UnsafeMutablePointer<Element>) {
     
-    _sanityCheck(
-      storage is _ContiguousArrayStorage<Element>, "Invalid array storage type")
-    
     let innerBuffer = _ContiguousArrayBuffer<Element>(
       count: count,
-      storage: unsafeDowncast(
-        storage, to: _ContiguousArrayStorage<Element>.self))
+      storage: storage)
     
     return (
       Array(