Always use LookupImplWitness instructions for symbolic witnesses (#5321)

We eliminate the `FacetAccessWitness` instruction, which would sometimes
immediately evaluate to a concrete `ImplWitness`, and sometimes remain
symbolic. This instruction is now replaced by `LookupImplWitness` in all
cases. To support the same use cases, when it is evaluated,
`LookupImplWitness` will look in the self value if it's a facet value,
and attempt to return a concrete `ImplWitness` from it before looking
for an `impl` statement.

The `LookupImplWitness` instruction's value is now canonical, even when
it evaluates to a symbolic `LookupImplWitness` instruction, by
canonicalizing the self value of the lookup query. This canonicalization
unwraps `FacetAccessType` and `FacetValue` instructions to get to an
underlying canonical facet value. However we must preserve and use the
non-canonical query while evaluating the instruction in order to look
for a concrete `ImplWitness` if the query self value was a concrete
`FacetValue`. The canonicalization ensures that symbolic witnesses
obtained from a facet value are compatible with those obtained from an
impl statement, as long as the self types originate from the same
canonical facet value though they may have been narrowed.

Member access now unconditionally does a `LookupImplWitness()`
operation, instead of only sometimes doing the lookup for a final impl
declaration.

`EvalImplLookupResult` is marked `[[nodiscard]]` so that we don't
construct it and forget to return it. This was a mistake made at one
point during the creation of this PR. And the `has_concrete_value()`
method no longer has a precondition that `has_value()` is true, since we
want to look for a concrete result only in the new use of
`EvalImplLookupResult` returned from lookup into the query self facet
value.

The TODO from `FacetAccessWitness` evaluation is addressed by ensuring
the index of the witness in the `FacetValue` comes from the required
interfaces of the `FacetValue`'s type, and that the type (a `FacetType`)
is the same facet type used in the query to construct the `FacetValue`'s
witness block. This is made possible by eliminating the
`FacetAccessWitness` indirection. The lookup into a `FacetValue` happens
while evaluating `LookupImplWitness` and it does so directly on the self
value. This gives a consistent view of the witness set and the facet
type, as they both come from the same instruction.

All of this with 400 less lines of code. :)

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/sem_ir/stringify.cpp

@@ -295,13 +295,6 @@ class Stringifier {
     step_stack_->PushInstId(inst.facet_value_inst_id);
   }
 
-  auto StringifyInst(SemIR::InstId /*inst_id*/, FacetAccessWitness inst)
-      -> void {
-    *out_ << "<witness for ";
-    step_stack_->Push(inst.facet_value_inst_id, ", interface ", inst.index,
-                      ">");
-  }
-
   auto StringifyInst(SemIR::InstId /*inst_id*/, FacetType inst) -> void {
     const FacetTypeInfo& facet_type_info =
         sem_ir_->facet_types().Get(inst.facet_type_id);
@@ -412,20 +405,7 @@ class Stringifier {
           lookup->query_specific_interface_id);
     }
 
-    if (auto witness =
-            sem_ir_->insts().TryGetAs<FacetAccessWitness>(impl_witness_id)) {
-      auto witness_type_id =
-          sem_ir_->insts().Get(witness->facet_value_inst_id).type_id();
-      auto facet_type = sem_ir_->types().GetAs<FacetType>(witness_type_id);
-      // TODO: Support != 1 interface better.
-      const auto& facet_type_info =
-          sem_ir_->facet_types().Get(facet_type.facet_type_id);
-      if (facet_type_info.extend_constraints.size() == 1) {
-        return facet_type_info.extend_constraints.front();
-      }
-    }
-
-    // TODO: Handle other cases.
+    // TODO: Handle ImplWitness.
     return std::nullopt;
   }
 
@@ -466,17 +446,17 @@ class Stringifier {
                         ")");
     }
 
-    if (auto witness =
-            sem_ir_->insts().TryGetAs<FacetAccessWitness>(witness_inst_id)) {
+    if (auto lookup =
+            sem_ir_->insts().TryGetAs<LookupImplWitness>(witness_inst_id)) {
       bool period_self = false;
       if (auto sym_name = sem_ir_->insts().TryGetAs<BindSymbolicName>(
-              witness->facet_value_inst_id)) {
+              lookup->query_self_inst_id)) {
         auto name_id =
             sem_ir_->entity_names().Get(sym_name->entity_name_id).name_id;
         period_self = (name_id == SemIR::NameId::PeriodSelf);
       }
       if (!period_self) {
-        step_stack_->PushInstId(witness->facet_value_inst_id);
+        step_stack_->PushInstId(lookup->query_self_inst_id);
       }
     } else {
       // TODO: Omit parens if not needed for precedence.