[moveOnly] Implement a new _copy function that performs an explicit copy value.

The key thing is that the move checker will not consider the explicit copy value
to be a copy_value that can be rewritten, ensuring that any uses of the result
of the explicit copy_value (consuming or other wise) are not checked.

Similar to the _move operator I recently introduced, this is a transparent
function so we can perform one level of specialization and thus at least be
generic over all concrete types.

lib/SILOptimizer/Transforms/AccessEnforcementReleaseSinking.cpp

@@ -175,6 +175,7 @@ static bool isBarrier(SILInstruction *inst) {
     case BuiltinValueKind::AssignTakeArray:
     case BuiltinValueKind::UnsafeGuaranteed:
     case BuiltinValueKind::Move:
+    case BuiltinValueKind::Copy:
     case BuiltinValueKind::UnsafeGuaranteedEnd:
     case BuiltinValueKind::CancelAsyncTask:
     case BuiltinValueKind::StartAsyncLet: