[OPENMP] Allow to use compound assignment operators.

Loop-based directives allow to use iterators as loop counters. Iterators are allowed to define their own operators. This patch allows to use compound assignment operators for iterators.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260957 91177308-0d34-0410-b5e6-96231b3b80d8