Detect functions that never return (and @noreturn annotation)

Some functions, such as error, never return control to the caller. This can be used to prove that a value is not nil, for example:

function my_function(input)
  local value = input.value or error('Missing value')
  -- The following line would currently raise a "Needs nil check" warning
  -- However, if it was nil at runtime, an error would have been raised and it would never get this far.
  return value:sub(1, 3)
end

It's not just error that can cause this, however. A custom function could be written that never returns as well (the most obvious way is that the function unconditionally calls error, but there may be other ways involving coroutines or functions implemented in C). A @noreturn annotation could be used to indicate this, similar to @nodiscard.

Issue 中没有指定文件或测试。首先跟踪 nil 检查警告以及现有的 @nodiscard 注解处理，然后确定应如何表示不返回的内置函数、自定义函数和 @noreturn；当示例不再发出警告且注解行为得到覆盖时，即表示完成。