Add imports of enclosing scopes. (#3575)

Namespaces are copied, which means also adding their name to the
underlying instruction. It happened not to be done previously; the name
was only in name lookup.

Since the only import supported right now is the default import,
functionality is limited; in the future I'll need to deal with namespace
vs package conflicts.

Tests of namespace imports are under "namespace" -- I figured this would
be best for scaling as more instructions get support.

This also improves some debugging-related output that I was trying to
use while trying to build the support.

---------

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

toolchain/check/handle_function.cpp

@@ -4,9 +4,11 @@
 
 #include "toolchain/check/context.h"
 #include "toolchain/check/convert.h"
+#include "toolchain/check/decl_name_stack.h"
 #include "toolchain/check/modifiers.h"
 #include "toolchain/parse/tree_node_location_translator.h"
 #include "toolchain/sem_ir/entry_point.h"
+#include "toolchain/sem_ir/ids.h"
 
 namespace Carbon::Check {
 
@@ -169,10 +171,8 @@ static auto BuildFunctionDecl(Context& context,
   // Create a new function if this isn't a valid redeclaration.
   if (!function_decl.function_id.is_valid()) {
     function_decl.function_id = context.functions().Add(
-        {.name_id =
-             name_context.state == DeclNameStack::NameContext::State::Unresolved
-                 ? name_context.unresolved_name_id
-                 : SemIR::NameId::Invalid,
+        {.name_id = name_context.name_id_for_new_inst(),
+         .enclosing_scope_id = name_context.enclosing_scope_id_for_new_inst(),
          .decl_id = function_decl_id,
          .implicit_param_refs_id = implicit_param_refs_id,
          .param_refs_id = param_refs_id,