Add the form ID to FormParamPattern (#6928)

This enables some nice simplifications, and it's also a step toward a
broader restructuring of binding and parameter patterns.

Assisted-by: Gemini 3.1 Pro via Antigravity

toolchain/check/thunk.cpp

@@ -127,12 +127,13 @@ static auto ClonePattern(Context& context, SemIR::SpecificId specific_id,
   }
 
   // Rebuild parameter.
-  if (param) {
+  if (param && new_pattern_id != SemIR::ErrorInst::InstId) {
     new_pattern_id = RebuildPatternInst<SemIR::AnyParamPattern>(
         context, param_id,
         {.kind = param->kind,
          .type_id = get_type(param_id),
-         .subpattern_id = new_pattern_id});
+         .subpattern_id = new_pattern_id,
+         .form_id = SemIR::ConstantId::None});
   }
 
   return new_pattern_id;