Forbid nested where inside a where expression (#7378)

This disallows building a facet type that contains another facet type
with non-extend constraints in it. Which in turn prevents the
possibility of introducing a different `.Self` into a facet type.

Eval can still insert a facet type with non-extend constraints, as we
only prevent it for `where` being written into the facet type. There is
a TODO in handle_where.cpp for this and some tests in
toolchain/check/testdata/facet/nested_facet_types_from_eval.carbon

toolchain/check/eval.cpp

@@ -3243,6 +3243,10 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
   Phase phase = Phase::Concrete;
   SemIR::FacetTypeInfo info;
 
+  if (inst.type_id() == SemIR::ErrorInst::TypeId) {
+    return SemIR::ErrorInst::ConstantId;
+  }
+
   // Note that these requirement instructions don't have a constant value. That
   // means we have to look for errors inside them, we can't just look to see if
   // their constant value is an error.