Track the instruction used to name the type and constraint in an impl. (#4368)

This is necessary in order to have access to the specific versions of
their constant values in a generic impl.

Stub out impl deduction.

toolchain/sem_ir/impl.cpp

@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/sem_ir/impl.h"
+
+#include "toolchain/sem_ir/file.h"
+
+namespace Carbon::SemIR {
+
+auto ImplStore::GetOrAddLookupBucket(const Impl& impl) -> LookupBucketRef {
+  auto self_id = sem_ir_.constant_values().GetConstantInstId(impl.self_id);
+  auto constraint_id =
+      sem_ir_.constant_values().GetConstantInstId(impl.constraint_id);
+  return LookupBucketRef(
+      *this, lookup_
+                 .Insert(std::pair{self_id, constraint_id},
+                         [] { return ImplOrLookupBucketId::Invalid; })
+                 .value());
+}
+
+}  // namespace Carbon::SemIR