Merge pull request #2182 from BuckleScript/expose_warning_message

[compiler] expose Warnings.message to make sure our super error message patch more robust

Author: bobzhang

vendor/ocaml/utils/warnings.mli

@@ -86,6 +86,7 @@ val backup: unit -> state
 val restore: state -> unit
 
 #if undefined BS_NO_COMPILER_PATCH then
+val message : t -> string 
 val number: t -> int
 val super_print : (t -> string) -> formatter -> t -> unit;;
 #end