Make Unsafe[Mutable]BufferPointer conform to BitwiseCopyable.

The Span interface requires this. It needs to be able to create dependencies on
UBP values without creating extra temporary copies.

This is only a temporary workaround for:

rdar://140291657 (ASTPrinter: print synthesized conformances on the type
definition, not as an extension)

Until that ASTPrinter bug is fixed, .swiftinterface files drop BitwiseCopyable
conformance on types that have conditionally ~Copyable generic parameters.

Author: atrick

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -110,6 +110,10 @@ public struct Unsafe${Mutable}BufferPointer<Element: ~Copyable>: Copyable {
 % end
 }
 
+// FIXME: The ASTPrinter should print this synthesized conformance.
+// rdar://140291657
+extension Unsafe${Mutable}BufferPointer: BitwiseCopyable where Element: ~Copyable {}
+
 @available(*, unavailable)
 extension Unsafe${Mutable}BufferPointer: Sendable where Element: ~Copyable {}