Form the contents of the witness table. (#3756)

Check that functions in an `impl` are declared properly. For now, simply
reuse the redeclaration checking logic. Longer-term, we need to check
that the signatures match in a more approximate way, after substituting
the `Self` type into the signature in the `interface`.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/function.cpp

@@ -173,6 +173,12 @@ static auto CheckRedecl(Context& context, const SemIR::Function& new_function,
   return true;
 }
 
+auto CheckFunctionRedecl(Context& context, SemIR::FunctionId new_function_id,
+                         SemIR::FunctionId prev_function_id) -> bool {
+  return CheckRedecl(context, context.functions().Get(new_function_id),
+                     context.functions().Get(prev_function_id));
+}
+
 auto MergeFunctionRedecl(Context& context, Parse::NodeId parse_node,
                          SemIR::Function& new_function,
                          SemIR::FunctionId prev_function_id, bool is_definition)