[OPENMP] Fix generation of the unique names for task reduction

variables.

If the task has reduction construct and this construct for some variable
requires unique threadprivate storage, we may generate different names
for variables used in taskgroup task_reduction clause and in task
  in_reduction clause. Patch fixes this problem.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@326827 91177308-0d34-0410-b5e6-96231b3b80d8