Initial scaffolding for building a witness table for an impl. (#3743)

Add an instruction to hold the witness table, along with a corresponding
type to keep things simpler. Add `check/impl.{h,cpp}` to house the new
logic. No checking of impls against interfaces is performed yet.

---------

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

toolchain/check/handle_impl.cpp

@@ -5,6 +5,7 @@
 #include "toolchain/check/context.h"
 #include "toolchain/check/convert.h"
 #include "toolchain/check/decl_name_stack.h"
+#include "toolchain/check/impl.h"
 #include "toolchain/check/modifiers.h"
 #include "toolchain/parse/typed_nodes.h"
 #include "toolchain/sem_ir/ids.h"
@@ -270,11 +271,14 @@ auto HandleImplDefinition(Context& context,
                           Parse::ImplDefinitionId /*parse_node*/) -> bool {
   auto impl_id =
       context.node_stack().Pop<Parse::NodeKind::ImplDefinitionStart>();
+
+  if (!context.impls().Get(impl_id).is_defined()) {
+    context.impls().Get(impl_id).witness_id =
+        BuildImplWitness(context, impl_id);
+  }
+
   context.inst_block_stack().Pop();
   context.decl_name_stack().PopScope();
-
-  // The impl is now fully defined.
-  context.impls().Get(impl_id).defined = true;
   return true;
 }