Use more compact storage for impl lookup buckets. (#4351)

If the bucket is of size zero or one, which is expected to be the common
case, then store it directly. For the remaining cases, use a side table
of lookup buckets.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/handle_impl.cpp

@@ -279,9 +279,9 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
       {.self_id = self_type_id, .constraint_id = constraint_type_id}};
 
   // Add the impl declaration.
-  auto& lookup_bucket = context.impls().GetOrAddLookupBucket(
+  auto lookup_bucket_ref = context.impls().GetOrAddLookupBucket(
       impl_info.self_id, impl_info.constraint_id);
-  for (auto prev_impl_id : lookup_bucket) {
+  for (auto prev_impl_id : lookup_bucket_ref) {
     if (MergeImplRedecl(context, impl_info, prev_impl_id)) {
       impl_decl.impl_id = prev_impl_id;
       break;
@@ -292,7 +292,7 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
   if (!impl_decl.impl_id.is_valid()) {
     impl_info.generic_id = FinishGenericDecl(context, impl_decl_id);
     impl_decl.impl_id = context.impls().Add(impl_info);
-    lookup_bucket.push_back(impl_decl.impl_id);
+    lookup_bucket_ref.push_back(impl_decl.impl_id);
   } else {
     FinishGenericRedecl(context, impl_decl_id,
                         context.impls().Get(impl_decl.impl_id).generic_id);