Gracefully handle a concrete ImplWitnessAccess in the type structure (#7214)

While this can only happen when some other error is taking place, we
should handle it gracefully and report a concrete (but unmatchable)
value in the type structure instead of CHECK-failing.

toolchain/check/testdata/impl/lookup/access.carbon

@@ -53,3 +53,31 @@ fn H(U:! type, V:! Z where .Z1 impls (Y where .Y1 = U)) {
   // query's.
   V as X(U);
 }
+
+// --- fail_concrete_impl_witness_access_in_type_structure.carbon
+library "[[@TEST_NAME]]";
+
+interface Z(T:! type) {
+  let Z1:! type;
+}
+interface Y {}
+
+class C;
+
+// Converting to this facet type requires a lookup for `C.(Z(C).Z1) as Y`. That
+// has a type structure that contains a fully concrete ImplWitnessAccess. But if
+// the lookup fails because there is no impl for `C as Z(C)`, then the access
+// remains in the type structure as is. This test ensures the type structure can
+// handle this edge case.
+fn F(unused T:! Z(.Self) where C impls (Z(C) where .Z1 impls Y)) {}
+
+fn G(T:! Z(.Self)) {
+  // CHECK:STDERR: fail_concrete_impl_witness_access_in_type_structure.carbon:[[@LINE+7]]:3: error: cannot convert type `T` that implements `Z(.Self)` into type implementing `Z(.Self) where C impls Z(C) and C.(Z(C).Z1) impls Y` [ConversionFailureFacetToFacet]
+  // CHECK:STDERR:   F(T);
+  // CHECK:STDERR:   ^~~~
+  // CHECK:STDERR: fail_concrete_impl_witness_access_in_type_structure.carbon:[[@LINE-6]]:1: note: while deducing parameters of generic declared here [DeductionGenericHere]
+  // CHECK:STDERR: fn F(unused T:! Z(.Self) where C impls (Z(C) where .Z1 impls Y)) {}
+  // CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  F(T);
+}

toolchain/sem_ir/type_iterator.cpp

@@ -254,10 +254,8 @@ auto TypeIterator::ProcessType(InstId inst_id) -> std::optional<Step> {
       // We return FacetValues as a separate iterative step, then also recurse
       // into the the self value being accessed.
       //
-      // Witness access of a concrete value would have evaluated to the accessed
-      // value, so we only see ImplWitnessAccess in a type when it's a symbolic
-      // value, which implies it contains a LookupImplWitness.
-      CARBON_CHECK(sem_ir_->constant_values().Get(inst_id).is_symbolic());
+      // If ImplWitnessAccess did not evaluate to some other value, it must
+      // contain a non-final witness.
       auto witness =
           sem_ir_->insts().GetAs<LookupImplWitness>(access.witness_id);
       // Recurse into symbolic ImplWitnessAccess, replacing it with the self