[C API] Add LLVMConstExactUDiv and LLVMBuildExactUDiv functions.

Summary:
These are analog to the existing LLVMConstExactSDiv and LLVMBuildExactSDiv
functions.

Reviewers: deadalnix, majnemer

Subscribers: majnemer, llvm-commits

Differential Revision: https://reviews.llvm.org/D25259

llvm-svn: 283269