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/pattern.cpp

@@ -171,7 +171,8 @@ auto AddParamPattern(Context& context, SemIR::LocId loc_id,
           loc_id, SemIR::AnyParamPattern{
                       .kind = param_pattern_kind,
                       .type_id = context.insts().Get(pattern_id).type_id(),
-                      .subpattern_id = pattern_id}));
+                      .subpattern_id = pattern_id,
+                      .form_id = SemIR::ConstantId::None}));
 
   return pattern_id;
 }