assign-type-mismatch not emitted when assigning a class to an array

Nobody has claimed this yet.

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

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Linux

### What is the issue affecting?

Type Checking

### Expected Behaviour

```lua
---@class a
local a = {}

---@class b
local b = {}

---@type a[]
local c = {
a,
b, // assign-type-mismatch emitted here
}
c = "" // assign-type-mismatch emitted here
c = b // assign-type-mismatch emitted here
c = a // assign-type-mismatch emitted here
```

### Actual Behaviour

```lua
---@class a
local a = {}

---@class b
local b = {}

---@type a[]
local c = {
a,
b, // assign-type-mismatch is correctly emitted here
}
c = "" // assign-type-mismatch is correctly emitted here
c = b // No assign-type-mismatch is emitted here. I would expect it to be emitted here.
c = a // No assign-type-mismatch is emitted here. I would expect it to be emitted here.
```

### Reproduction steps

.luarc.json:

```
{
"$schema": "https://raw.githubusercontent.com/LuaLS/vscode-lua/master/setting/schema.json",
"runtime.version": "Lua 5.1"
}
```

### Additional Notes

_No response_

### Log File

_No response_

Reproduce the report using the supplied .luarc.json and Lua snippet, checking the three assignment cases involving the a[] annotation. Trace the type-checking path responsible for assign-type-mismatch diagnostics and compare class assignments with the already-diagnosed string assignment and array literal element. Done means mismatched class assignments to the array are diagnosed without changing the existing diagnostics.

Written by the indexing model from the issue text.