Outputting a @ccallable function to an object file broken on master

[KristofferC]

MWE from test failure in PackageCompiler.jl

# callable.jlj
Base.@ccallable function f()::Cint
    return 0
end

run(`$(Base.julia_cmd()) --startup=no --output-o=sys.o ccallable.jl`)

prints:

Assertion failed: (Val && "isa<> used on a null pointer"), function doit, file /Users/kristoffercarlsson/julia/usr/include/llvm/Support/Casting.h, line 105.

signal (6): Abort trap: 6
in expression starting at /Users/kristoffercarlsson/julia/ccallable.jl:1
__pthread_kill at /usr/lib/system/libsystem_kernel.dylib (unknown line)

[KristofferC]

Bisected to #25984, cc @vtjnash