Distinguish between name IDs and string IDs in the type system. (#3341)

Add a `NameId` that is effectively just a wrapper around a `StringId`,
with
some additional predefined values for names that don't correspond to
strings, such as the name of `self` or the function's return slot.

toolchain/check/handle_struct.cpp

@@ -41,7 +41,7 @@ auto HandleStructFieldUnknown(Context& context, Parse::Node parse_node)
 auto HandleStructFieldValue(Context& context, Parse::Node parse_node) -> bool {
   auto [value_parse_node, value_inst_id] =
       context.node_stack().PopExpressionWithParseNode();
-  IdentifierId name_id = context.node_stack().Pop<Parse::NodeKind::Name>();
+  SemIR::NameId name_id = context.node_stack().Pop<Parse::NodeKind::Name>();
 
   // Store the name for the type.
   context.args_type_info_stack().AddInst(SemIR::StructTypeField{