fix: missing pc map for empty functions (#3202)

the IR generated inside of a function (that is not generated in stmt.py
or expr.py) was not annotated with the source pos. this annotates the
root node of the function with the source pos as the fallback.

vyper/codegen/function_definitions/external_function.py

@@ -214,4 +214,4 @@ def generate_ir_for_external_function(code, sig, context, skip_nonpayable_check)
         # TODO rethink this / make it clearer
         ret[-1][-1].append(func_common_ir)
 
-    return IRnode.from_list(ret)
+    return IRnode.from_list(ret, source_pos=getpos(sig.func_ast_code))