pub fn add_local(&mut self, ty: &Type, func: FunctionId) -> Symbol {
        let sym = self.sym_gen.new_symbol(format!("fn{}_tmp", func).as_str());
        self.funcs[func].locals.insert(sym.clone(), ty.clone());
        sym
    }
Add a local variable of the given type and return a symbol for it.
weld/src/sir/mod.rs