Refactor BindName to support tracking the enclosing scope. (#3566)

This is a step towards adding enclosing scopes for imports. It creates
an indirection for all bind names.

We discussed specializing for bindings that are in function scope (i.e.,
not a useful enclosing scope for imports or diagnostics). However, the
thought is to go ahead with this singular approach for now, and only
change structure if it's a performance issues so that we have
incrementally fewer instructions to handle.

toolchain/check/context.cpp

@@ -144,9 +144,11 @@ auto Context::AddPackageImports(Parse::NodeId import_node,
   // Add a name for formatted output. This isn't used in name lookup in order
   // to reduce indirection, but it's separate from the Import because it
   // otherwise fits in an Inst.
+  auto bind_name_id = bind_names().Add(
+      {.name_id = name_id, .enclosing_scope_id = SemIR::NameScopeId::Package});
   AddInst(SemIR::BindName{.parse_node = import_node,
                           .type_id = type_id,
-                          .name_id = name_id,
+                          .bind_name_id = bind_name_id,
                           .value_id = inst_id});
 }