embedded stdlib: make the ManagedBuffer initializer public in the embedded stdlib

Otherwise derived classes cannot be specialized.

Fixes a compile time error when using ManagedBuffer in embedded mode.
rdar://122414669

Author: eeckstein

stdlib/public/core/ManagedBuffer.swift

@@ -44,13 +44,21 @@ open class ManagedBuffer<Header, Element> {
   /// reading the `header` property during `ManagedBuffer.create` is undefined.
   public final var header: Header
 
+  #if $Embedded
+  // In embedded mode this initializer has to be public, otherwise derived
+  // classes cannot be specialized.
+  public init(_doNotCallMe: ()) {
+    _internalInvariantFailure("Only initialize these by calling create")
+  }
+  #else
   // This is really unfortunate. In Swift 5.0, the method descriptor for this
   // initializer was public and subclasses would "inherit" it, referencing its
   // method descriptor from their class override table.
   @usableFromInline
   internal init(_doNotCallMe: ()) {
     _internalInvariantFailure("Only initialize these by calling create")
   }
+  #endif
 
   @inlinable
   deinit {}

test/embedded/managed-buffer.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-emit-ir %s -module-name=main -enable-experimental-feature Embedded | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+// CHECK: @"$s4main8MyBufferCN" = {{.*global.*}} <{ ptr @"$ss13ManagedBufferCySis5UInt8VGN", ptr @"$s4main8MyBufferCfD", ptr @"$s4main8MyBufferC12_doNotCallMeACyt_tcfC" }>
+// CHECK: @"$ss13ManagedBufferCySis5UInt8VGN" = {{.*global.*}} <{ ptr null, ptr @"$ss13ManagedBufferCfDSi_s5UInt8VTg5", ptr @"$ss13ManagedBufferC12_doNotCallMeAByxq_Gyt_tcfCSi_s5UInt8VTg5" }>
+final public class MyBuffer: ManagedBuffer<Int, UInt8> {
+}
+