[JuliaLowering] Add remap for assigned-to arguments (#60551)
Nearly 1:1 with the usage of `arg-map` in flisp (was noted as TODO in
JuliaLowering)
This missing functionality was surprisingly hard to observe since slots
are erased in our optimization pipeline, making it impossible for
codegen to complain about them. Nonetheless, macros can crash if we do
not perform this re-write.
Resolves https://github.com/JuliaLang/JuliaLowering.jl/issues/129
