[OCaml] Fix undefined reference to LLVMDumpType() with NDEBUG

[mgorny]

Summary

Account for the possibility of LLVMDumpType() not being available with
NDEBUG in the OCaml bindings. If it is not built into LLVM, make
the dump function raise an exception.

Since rL293359, the dump functions are built only if either NDEBUG is
not defined, or LLVM_ENABLE_DUMP is defined. As a result, if the dump
functions are not built in LLVM, the dynamic OCaml libraries fail to
load due to undefined LLVMDumpType symbol.

[whitequark]

bindings/ocaml/llvm/llvm_ocaml.c

343

caml_raise_with_arg(*caml_named_value("Llvm.FeatureDisabled"), caml_copy_string("dump"))

[whitequark]

bindings/ocaml/llvm/llvm_ocaml.c

344

It's defined as

CAMLextern value caml_copy_string (char const *);

you can just use caml_copy_string("dump").

[whitequark]

LGTM

bindings/ocaml/llvm/llvm.ml

 type lluse
 type llbasicblock
 type llbuilder
 type llattrkind
 type llattribute
 type llmemorybuffer
 type llmdkind
 
+exception FeatureDisabled of string
+
+let () = Callback.register_exception "Llvm.FeatureDisabled" (FeatureDisabled "")
+
 module TypeKind = struct
   type t =
   | Void
   | Half
   | Float
   | Double
   | X86fp80
   | Fp128

bindings/ocaml/llvm/llvm.mli

(** [After b] and [At_start a] specify positions from the end of the ['b] list
     lists maintained by the LLVM IR. *)
 type ('a, 'b) llrev_pos =
 | At_start of 'a
 | After of 'b
 
 
 (** {6 Exceptions} *)
 
+exception FeatureDisabled of string
+
 exception IoError of string
 
 
 (** {6 Global configuration} *)
 
 (** [enable_pretty_stacktraces ()] enables LLVM's built-in stack trace code.
     This intercepts the OS's crash signals and prints which component of LLVM
     you were in at the time of the crash. *)

bindings/ocaml/llvm/llvm_ocaml.c

 
 /* lltype -> llcontext */
 CAMLprim LLVMContextRef llvm_type_context(LLVMTypeRef Ty) {
   return LLVMGetTypeContext(Ty);
 }
 
 /* lltype -> unit */
 CAMLprim value llvm_dump_type(LLVMTypeRef Val) {
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
   LLVMDumpType(Val);
+#else
+  char Message[] = "dump";
+  llvm_raise(*caml_named_value("Llvm.FeatureDisabled"), Message);
+#endif
   return Val_unit;
 }
 
 /* lltype -> string */
 CAMLprim value llvm_string_of_lltype(LLVMTypeRef M) {
   CAMLparam0();
   CAMLlocal1(TypeStr);
   char* TypeCStr;