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/eval.cpp

@@ -329,6 +329,9 @@ auto TryEvalInst(Context& context, SemIR::InstId inst_id, SemIR::Inst inst)
       return RebuildIfFieldsAreConstant(context, inst,
                                         &SemIR::BoundMethod::object_id,
                                         &SemIR::BoundMethod::function_id);
+    case SemIR::InterfaceWitness::Kind:
+      return RebuildIfFieldsAreConstant(context, inst,
+                                        &SemIR::InterfaceWitness::table_id);
     case SemIR::PointerType::Kind:
       return RebuildIfFieldsAreConstant(context, inst,
                                         &SemIR::PointerType::pointee_id);