[JuliaLowering] fix simple `(const ident val)` (#61374)
Lowering is expected to accept pre-desugared `(const ident val)` in
addition to the standard `(const (= ident val))` produced by parsing.
Fixes half of https://github.com/JuliaLang/JuliaLowering.jl/issues/149.
To resolve it fully, I still need to make globalrefs work as
identifiers, which I'll get to next.