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.

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

toolchain/check/handle_function.cpp

@@ -97,7 +97,7 @@ static auto BuildFunctionDeclaration(Context& context, bool is_definition)
         {.name_id = name_context.state ==
                             DeclarationNameStack::NameContext::State::Unresolved
                         ? name_context.unresolved_name_id
-                        : SemIR::StringId(SemIR::StringId::InvalidIndex),
+                        : StringId::Invalid,
          .param_refs_id = param_refs_id,
          .return_type_id = return_type_id,
          .return_slot_id = return_slot_id});