Incorrect field type calculation when field name in variable

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

Visual Studio Code Extension (sumneko.lua) v3.11.0

### Which OS are you using?

Linux

### What is the issue affecting?

Type Checking

### Reproduction steps

```lua
---@diagnostic disable: unused-local

---@class Class
local Class = { __type = "Class" }

local field = "some_string" -- field: string = "some_string"

do
local c = {} ---@type Class

-- OK
local f1 = c["some_string"] -- f1: unknown
local f2 = c[field] -- f2: unknown
end

function Class:f()
-- OK
local f1 = self["some_string"] -- f1: unknown

-- BUT
local f2 = self[field] -- f2: string = "Class"
end

do
-- OK
local f1 = Class["some_string"] -- f1: unknown

-- BUT
local f2 = Class[field] -- f2: string = "Class"
end

```

Start by reproducing the type-checking discrepancy in the Lua language server with the code sample, comparing literal indexing with indexing through the `field` variable. Trace the field-type inference entry point and add a regression test for the shown cases; done means variable-based indexing no longer reports `Class` where literal indexing reports `unknown`.

Written by the indexing model from the issue text.