relocate_gvs!: keep Type-valued globals as identity tokens

[vchuravy]

Problem

relocate_gvs! decides whether to materialize a global's value with:

if isbitstype(typeof(obj)) && sizeof(obj) > 0 && !(obj isa Bool)

Union{} slips through this guard. typeof(Union{}) is Core.TypeofBottom, an isbits singleton, so isbitstype(typeof(obj)) is true — but sizeof(Union{}) throws (The empty type does not have a definite size since it does not have instances.) because the type is uninhabited.

This surfaces when differentiating ordinary code with Enzyme.jl on Julia 1.12 (GPUCompiler v2 path): the primal module carries a julia.constgv global pointing at Union{}, and relocate_gvs! crashes while baking it in:

The empty type does not have a definite size since it does not have instances.
Stacktrace:
 [1] sizeof(x::Type)
 [2] relocate_gvs!(mod::LLVM.Module, gv_to_value::Dict{String, Ptr{Nothing}})
     @ GPUCompiler .../src/jlgen.jl:800
 [3] emit_llvm(job::GPUCompiler.CompilerJob)

Fix

Exclude obj isa Type from materialization. Type objects — like any other non-isbits object — keep their host address as an identity token (baked inttoptr), which renders the module session-dependent, exactly as intended for objects whose identity matters.

Test

Adds a relocate_gvs! unit test for a slot pointing at Union{}, asserting it is baked as an address rather than fed to materialize_box!. Verified the test crashes on master and passes with this change.

🤖 Generated with Claude Code