[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/SIL/Utils/MemAccessUtils.cpp

@@ -2191,9 +2191,9 @@ static void visitBuiltinAddress(BuiltinInst *builtin,
       visitor(&builtin->getAllOperands()[2]);
       return;
 
-    // This consumes its second parameter (the arg) and takes/places that value
-    // into the first parameter (the result).
+    // These effect both operands.
     case BuiltinValueKind::Move:
+    case BuiltinValueKind::Copy:
       visitor(&builtin->getAllOperands()[1]);
       return;