Use GlobalRef of Core.CodeInfo in @generated

[devmotion]

This PR addresses https://github.com/JuliaLang/julia/pull/40778/files#r784416018 and replaces the reference to Core.CodeInfo in @generated with a GlobalRef.

The current implementation caused errors in Turing (TuringLang/Turing.jl#1756) which collected core AD parts in a Core submodule (not anymore, it's renamed to Essential now since it's not the first time this caused problems: simonster/Reexport.jl#33 (comment)).

Currently the PR is still missing some tests. It's a bit embarrassing but I did not manage to come up with a MWE that would be fixed by this PR. I tried things like

module M
module Core
@generated f(x) = :(2 * x)
end
end

M.Core.f(3)

but they all seemed to work fine. Maybe someone has a suggestion how I can reproduce the bug and ensure that Core in @generated refers to the submodule (or something else) without CodeInfo?

[simeonschaub]

Here's a test case that currently errors on nightly:

@test @eval Module() begin
    Core = 1
    @generated f() = 1
    f() == 1
end

[devmotion]

🤦 I realized I couldn't come up with a MWE because I used Julia 1.7.1 instead of 1.6.5 or master.

[devmotion]

I added a fix for arguments of name tmp as well and added both tests (added you as a commit co-author @simeonschaub). I used gensym since it seemed easier.

[simeonschaub]

Thanks! That's a great first contribution!

[simeonschaub]

Ah, I actually meant to approve this...