[OpenMP] Add options to change Attributor max iterations in OpenMPOpt

This patch adds a new command line option `openmp-opt-max-iterations`
that controls the maximum number of iterations the attributor will run
for when compiling OpenMP target device code. This patch also adds a
remark to indicate when the attributor failed because it did not run
for enough iterations.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D110749