How to make a third-party member function as deprecated

还没有人认领这个 Issue。

### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Windows

### What is the issue affecting?

Diagnostics/Syntax Checking

### Expected Behaviour

```lua
---@deprecated
M.add = M.add
```
The left one should have marked as deprecated immediately when used such as ~u:add~

### Actual Behaviour

1. it not works first.
2. and works unexpectedly.

### Reproduction steps

1. Write the code
```lua
-- File A

---@class A
local A = {}

---@param attr string
function A.add(attr)
end

-- File B

---@class A
local B = A

---@deprecated
B.add = A.add

B.add('some attr')
```
We'll find that B.add havn't marked as deprecated
2. Keep write the followed
```lua
-- File C

---@class A
---@field add fun(attr: string)
local C = A
```

### Additional Notes

My original intention was to mark a third-party member function as deprecated.
As there was no straightforward way, I think, to do it, I had to use a lot of tricks, which led to more subsequent problems.

### Log File

_No response_

从 issue 中的 File A、File B 和 File C 复现开始，重点关注诊断在已弃用赋值之后如何标记 B.add。在 Visual Studio Code 扩展中运行复现，并将观察到的诊断与针对 B.add 的预期弃用警告进行比较。

由索引模型根据 Issue 内容生成。