Add value store to be shared across compile stages. (#3311)

This updates lexing to use the data. I'll do checking separately, just
to split changes.

Note the ValueStore structure is also set up such that SemIR::File can
use it for other fields.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

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_node_id] =
       context.node_stack().PopExpressionWithParseNode();
-  SemIR::StringId name_id = context.node_stack().Pop<Parse::NodeKind::Name>();
+  StringId name_id = context.node_stack().Pop<Parse::NodeKind::Name>();
 
   // Store the name for the type.
   context.args_type_info_stack().AddNode(SemIR::StructTypeField(