[NFC][OCaml] Fix implicit declaration compilation warnings

Summary:
2dd4f35c7fc75639920ebc473a4a57ea91864c10 moved LLVMAddLowerSwitchPass
and LLVMAddPromoteMemoryToRegisterPass declarations from
llvm-c/Transforms/Scalar.h to llvm-c/Transforms/Utils.h

Reviewers: whitequark

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52214

llvm-svn: 342436