Adding Clflags.debug before main loop, as in Toploop

[ysalmon]

An old ocaml commit moved Clflags.debug := true from the module initalisation to their loop function, which we do not call.

So this adds the same just before calling the main (recursive) loop function in utop, both in normal and emacs mode.
Closes #501.

And then we get good backtraces :

─( 09:21:33 )─< command 0 >──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # let rec f (h::t) = h + f t;;

Line 1, characters 10-16:
Warning 8 [partial-match]: this pattern-matching is not exhaustive.
Here is an example of a case that is not matched:
[]

val f : int list -> int = <fun>
─( 09:21:33 )─< command 1 >──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # let g l = f (5::l);;
val g : int list -> int = <fun>
─( 09:21:42 )─< command 2 >──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # g [6;3;8];;
Exception: Match_failure ("//toplevel//", 1, 10).
Raised at f in file "//toplevel//", line 1, characters 10-16
Called from f in file "//toplevel//", line 1, characters 23-26
Called from f in file "//toplevel//", line 1, characters 23-26
Called from f in file "//toplevel//", line 1, characters 23-26
Called from f in file "//toplevel//", line 1, characters 23-26
Called from <unknown> in file "//toplevel//", line 1, characters 0-9
Called from Topeval.load_lambda in file "toplevel/byte/topeval.ml", line 93, characters 4-14

[Octachron]

The changes looks good to me.

[Octachron]

Merged, this should be part of the upcoming release of utop in preparation of the release of OCaml 5.4.0 .