Fix singleton parameters in overloaded functions

[maffeus]

• Fixes Overloaded functions with singleton parameters error incorrectly in new type solver #1691
• Fixes String literals aren't refined to singleton types in function overloads #1589

[maffeus]

This code snippet is working now, and you get autocompletion with luau-lsp too (which was broken previously).

--[[
    local date: {
        day: number,
        hour: number,
        isdst: boolean,
        ... 6 more ...
    }
]]
local date = os.date("*t", 0)