[stdlib] Suppress Unicode : _BitwiseCopyable.

Mark it always unavailable per LWG feedback.

Author: nate-chandler

stdlib/public/core/Unicode.swift

@@ -674,3 +674,6 @@ public func transcode<Input, InputEncoding, OutputEncoding>(
 @frozen
 public enum Unicode {}
 
+@available(*, unavailable)
+extension Unicode : _BitwiseCopyable {}
+

test/Sema/bitwise_copyable_2.swift

@@ -38,3 +38,5 @@ func passInternalUsableStruct(_ s: InternalUsableStruct) { take(s) } // expected
 func passMemoryLayout<T>(_ m: MemoryLayout<T>) { take(m) } // expected-error{{conformance_availability_unavailable}}
 
 func passCommandLine(_ m: CommandLine) { take(m) } // expected-error{{conformance_availability_unavailable}}
+
+func passUnicode(_ m: Unicode) { take(m) } // expected-error{{conformance_availability_unavailable}}