[OCaml] Add GlobalIFunc value kind to OCaml API

Summary:
The GlobalIFunc value kind has not yet been added to the OCaml
API. This patch only extends the enum, so that e.g. classify_value
will not crash. No support for manipulating or building GlobalIFuncs
is added at this point.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52198

llvm-svn: 342429