[BitwiseCopyable] Remove gating from protocol decl

It's harmless to have the protocol defined in the swiftinterface for
older compilers and removing the gating permits the gating to be omitted
from conformances

Author: nate-chandler

stdlib/public/core/Misc.swift

@@ -182,6 +182,4 @@ func _rethrowsViaClosure(_ fn: () throws -> ()) rethrows {
 @_marker public protocol Escapable {}
 #endif
 
-#if $BitwiseCopyable
 @_marker public protocol _BitwiseCopyable {}
-#endif

stdlib/public/core/Optional.swift

@@ -783,6 +783,4 @@ extension Optional: _ObjectiveCBridgeable {
 
 extension Optional: Sendable where Wrapped: Sendable { }
 
-#if $BitwiseCopyable
 extension Optional: _BitwiseCopyable where Wrapped: _BitwiseCopyable { }
-#endif