[OCaml] Llvm_linker: do not use external in module interface

Workaround for an OCaml bug:
http://caml.inria.fr/mantis/view.php?id=4166

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194057 91177308-0d34-0410-b5e6-96231b3b80d8
[OCaml] Properly tag the custom operations of Llvm.llbuilder

All other custom operations tags have LLVM prefix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194058 91177308-0d34-0410-b5e6-96231b3b80d8