Look outside constant values for designators in where constraints (#7367)

The constant value may lose the designator during eval, such as an
`ImplWitnessAccess` that resolves to some concrete type. Look in the
non-canonical instructions instead.

toolchain/check/handle_alias.cpp

@@ -35,8 +35,16 @@ auto HandleParseNode(Context& /*context*/,
 auto HandleParseNode(Context& context, Parse::AliasId /*node_id*/) -> bool {
   auto [expr_node, expr_id] = context.node_stack().PopExprWithNodeId();
 
-  auto name_context = context.decl_name_stack().FinishName(
-      PopNameComponentWithoutParams(context, Lex::TokenKind::Alias));
+  bool diagnosed_params = false;
+  auto name_context =
+      context.decl_name_stack().FinishName(PopNameComponentWithoutParams(
+          context, Lex::TokenKind::Alias, &diagnosed_params));
+  if (diagnosed_params) {
+    // If the alias had generic parameters, they could be SymbolicBindings which
+    // then appear in the `expr_id`. Using those elsewhere is then invalid and
+    // creates invalid states in the toolchain.
+    expr_id = SemIR::ErrorInst::InstId;
+  }
 
   DiscardGenericDecl(context);