[stdlib] Suppress MemoryLayout : _BitwiseCopyable.

Mark it always unavailable per LWG feedback.

Author: nate-chandler

stdlib/public/core/MemoryLayout.swift

@@ -76,6 +76,9 @@ public enum MemoryLayout<T> {
   }
 }
 
+@available(*, unavailable)
+extension MemoryLayout : _BitwiseCopyable {}
+
 extension MemoryLayout {
   /// Returns the contiguous memory footprint of the given instance.
   ///

test/Sema/bitwise_copyable_2.swift

@@ -1,5 +1,4 @@
 // RUN: %target-typecheck-verify-swift                       \
-// RUN:     -disable-availability-checking                   \
 // RUN:     -enable-experimental-feature NonescapableTypes   \
 // RUN:     -enable-experimental-feature BitwiseCopyable     \
 // RUN:     -enable-builtin-module                           \
@@ -35,3 +34,5 @@ func take<T : _BitwiseCopyable>(_ t: T) {}
 
 func passInternalUsableStruct(_ s: InternalUsableStruct) { take(s) } // expected-error{{type_does_not_conform_decl_owner}}
                                                                      // expected-note@-8{{where_requirement_failure_one_subst}}
+
+func passMemoryLayout<T>(_ m: MemoryLayout<T>) { take(m) } // expected-error{{conformance_availability_unavailable}}