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/name_component.cpp

@@ -81,8 +81,8 @@ auto PopNameComponent(Context& context, SemIR::InstId return_pattern_id)
 
 // Pop the name of a declaration from the node stack, and diagnose if it has
 // parameters.
-auto PopNameComponentWithoutParams(Context& context, Lex::TokenKind introducer)
-    -> NameComponent {
+auto PopNameComponentWithoutParams(Context& context, Lex::TokenKind introducer,
+                                   bool* diagnosed_params) -> NameComponent {
   NameComponent name = PopNameComponent(context);
   if (name.call_params_id.has_value()) {
     CARBON_DIAGNOSTIC(UnexpectedDeclNameParams, Error,
@@ -95,6 +95,9 @@ auto PopNameComponentWithoutParams(Context& context, Lex::TokenKind introducer)
                            UnexpectedDeclNameParams, introducer);
 
     name.call_params_id = SemIR::InstBlockId::None;
+    if (diagnosed_params) {
+      *diagnosed_params = true;
+    }
   }
   return name;
 }