lowering: Disallow splatting in non-final default value

Pop quiz: Do you know what the following will do?
```
julia> function g1(a=(1,2)..., b...=3)
    b
end

julia> g1()

julia> function g2(a=(1,2)..., b=3, c=4)
    (b, c)
end

julia> g2()

julia> function g3(a=(1,2)..., b=3, c...=4)
    (b, c)
end

julia> g3()

julia> g3(1)
```

I don't either and I don't think it's particularly well defined.
Splatting a default argument makes sense on the last argument,
which can be a vararg, and it is desirable to be able to specify
the default for the whole varargs tuple at once (although arguably
that should just be the non-`...` behavior, but that'd be too
breaking a change). Ref #50518. However, for other arguments,
there isn't really a sensible semantic meaning. This PR disallows
this in lowering. This is technically a minor change, but I doubt
anybody is using this. Splatting in default values wasn't really
ever supposed to work anyway, it just happened to fall out of
our lowering.

src/julia-syntax.scm

@@ -662,7 +662,10 @@
                       (vals   (list-tail dfl n))
                       (absent (list-tail opt n)) ;; absent arguments
                       (body
-                       (if (any (lambda (defaultv)
+                       (if (any (lambda (val) (and (pair? val) (eq? (car val) '...))) (butlast vals))
+                         ;; Forbid splat in all but the final default value
+                         (error "Splatting in non-final positional default")
+                         (if (any (lambda (defaultv)
                                   ;; does any default val expression...
                                   (contains (lambda (e)
                                               ;; contain "e" such that...
@@ -681,7 +684,7 @@
                            ;; otherwise add all
                            `(block
                              ,@prologue
-                             (call ,(arg-name (car req)) ,@(map arg-name (cdr passed)) ,@vals)))))
+                             (call ,(arg-name (car req)) ,@(map arg-name (cdr passed)) ,@vals))))))
                  (method-def-expr- name sp passed body)))
              (iota (length opt)))
       ,(method-def-expr- name sparams overall-argl body rett))))

test/syntax.jl

@@ -3507,3 +3507,23 @@ let x = 1 => 2
     @test_throws ErrorException @eval a => b = 2
     @test_throws "function Base.=> must be explicitly imported to be extended" @eval a => b = 2
 end
+
+# Splatting in non-final default value (Ref #50518)
+for expr in (quote
+    function g1(a=(1,2)..., b...=3)
+        b
+    end
+end,quote
+    function g2(a=(1,2)..., b=3, c=4)
+        (b, c)
+    end
+end,quote
+    function g3(a=(1,2)..., b=3, c...=4)
+        (b, c)
+    end
+end)
+    let exc = try eval(expr); catch exc; exc end
+        @test isa(exc, ErrorException)
+        @test startswith(exc.msg, "syntax: Splatting in non-final positional default")
+    end
+end