lowering: Allow chaining of >: in where (#57554)

This change removes an inconsistency between `>:` and `<:`. We were
parsing `where {A>:B>:C}` forms, but not recognizing them in lowering.

Before:
```
julia> Vector{T} where Int<:T<:Number
Vector{T} where Int64<:T<:Number

julia> Vector{T} where Number>:T>:Int
ERROR: syntax: invalid bounds in "where" around REPL[14]:1
```

After:
```
julia> Vector{T} where Number>:T>:Int
Vector{T} where Int64<:T<:Number
```

Author: mlechu

src/julia-syntax.scm

@@ -194,10 +194,14 @@
   (cond ((atom? e) (list (check-sym e) #f #f))
         ((eq? (car e) 'var-bounds)  (cdr e))
         ((and (eq? (car e) 'comparison) (length= e 6))
-         (cons (check-sym (cadddr e))
-               (cond ((and (eq? (caddr e) '|<:|) (eq? (caddr (cddr e)) '|<:|))
-                      (list (cadr e) (last e)))
-                     (else (error "invalid bounds in \"where\"")))))
+         (let* ((lhs (list-ref e 1))
+                (rel (list-ref e 2))
+                (t (check-sym (list-ref e 3)))
+                (rel-same (eq? rel (list-ref e 4)))
+                (rhs (list-ref e 5)))
+           (cond ((and rel-same (eq? rel '|<:|)) (list t lhs rhs))
+                 ((and rel-same (eq? rel '|>:|)) (list t rhs lhs))
+                 (else (error "invalid bounds in \"where\"")))))
         ((eq? (car e) '|<:|)
          (list (check-sym (cadr e)) #f (caddr e)))
         ((eq? (car e) '|>:|)

test/syntax.jl

@@ -1296,6 +1296,10 @@ let args = (Int, Any)
     @test >:(reverse(args)...)
 end
 
+# Chaining of <: and >: in `where`
+@test isa(Vector{T} where Int<:T<:Number, UnionAll)
+@test isa(Vector{T} where Number>:T>:Int, UnionAll)
+
 # issue #25947
 let getindex = 0, setindex! = 1, colon = 2, vcat = 3, hcat = 4, hvcat = 5
     a = [10,9,8]