Always form a ConstType instruction for const. (#6341)

Do this even if the operand is a `ConstType` instruction. This better
preserves the source form of the type, and avoids a special case.
Repeated `const`s are already flattened in constant evaluation, and this
special case also didn't prevent forming a `ConstType` whose operand is
`const` in general, only cases where the operand happens to literally be
a `ConstType` instruction.

This reverts commit eed21f6439.

toolchain/check/handle_operator.cpp

@@ -286,8 +286,6 @@ auto HandleParseNode(Context& context, Parse::PrefixOperatorConstId node_id)
                       "`const` applied repeatedly to the same type has no "
                       "additional effect");
     context.emitter().Emit(node_id, RepeatedConst);
-    context.node_stack().Push(node_id, value_id);
-    return true;
   }
   auto inner_type = ExprAsType(context, node_id, value_id);
   AddInstAndPush<SemIR::ConstType>(