ManagedBuffer capacity is unavailable on OpenBSD.

On OpenBSD, malloc introspection (e.g., malloc_usable_size or
malloc_size) is not provided by the platform allocator. Since allocator
introspection is currently a load-bearing piece of functionality for
ManagedBuffer and ManagedBufferPointer, pending any API changes, as a
stopgap measure, this commit marks methods in ManagedBuffer and
ManagedBufferPointer calling _swift_stdlib_malloc_size and methods
dependent thereon unavailable on OpenBSD.

This may induce some compatibility issues for some files, but at least
this change ensures that we can get stdlib to build on this platform
until the evolution process addresses this problem more thoroughly.

Author: 3405691582

stdlib/public/core/BridgingBuffer.swift

@@ -25,6 +25,7 @@ internal final class __BridgingBufferStorage
 internal typealias _BridgingBuffer
   = ManagedBufferPointer<_BridgingBufferHeader, AnyObject>
 
+@available(OpenBSD, unavailable, message: "malloc_size is unavailable.")
 extension ManagedBufferPointer
 where Header == _BridgingBufferHeader, Element == AnyObject {
   internal init(_ count: Int) {

stdlib/public/core/ManagedBuffer.swift

@@ -83,6 +83,7 @@ extension ManagedBuffer {
   /// idea to store this information in the "header" area when
   /// an instance is created.
   @inlinable
+  @available(OpenBSD, unavailable, message: "malloc_size is unavailable.")
   public final var capacity: Int {
     let storageAddr = UnsafeMutableRawPointer(Builtin.bridgeToRawPointer(self))
     let endAddr = storageAddr + _swift_stdlib_malloc_size(storageAddr)
@@ -197,6 +198,7 @@ public struct ManagedBufferPointer<Header, Element> {
   ///   properties.  The `deinit` of `bufferClass` must destroy its
   ///   stored `Header` and any constructed `Element`s.
   @inlinable
+  @available(OpenBSD, unavailable, message: "malloc_size is unavailable.")
   public init(
     bufferClass: AnyClass,
     minimumCapacity: Int,
@@ -329,6 +331,7 @@ extension ManagedBufferPointer {
   /// idea to store this information in the "header" area when
   /// an instance is created.
   @inlinable
+  @available(OpenBSD, unavailable, message: "malloc_size is unavailable.")
   public var capacity: Int {
     return (
       _capacityInBytes &- ManagedBufferPointer._elementOffset
@@ -431,6 +434,7 @@ extension ManagedBufferPointer {
 
   /// The actual number of bytes allocated for this object.
   @inlinable
+  @available(OpenBSD, unavailable, message: "malloc_size is unavailable.")
   internal var _capacityInBytes: Int {
     return _swift_stdlib_malloc_size(_address)
   }

test/Interpreter/generic_ref_counts.swift

@@ -1,5 +1,6 @@
 // RUN: %target-run-simple-swift | %FileCheck %s
 // REQUIRES: executable_test
+// XFAIL: OS=openbsd
 
 import Swift
 

test/stdlib/ManagedBuffer.swift

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 // RUN: %target-run-simple-swift
 // REQUIRES: executable_test
+// XFAIL: OS=openbsd
 
 import StdlibUnittest