Reduce perf impact of redundant ittnotify calls

Improved performance of ittnotify calls by request from ittnotify
owner: calls to __itt_string_handle_create made unique (it was
called multiple times).

Patch by Andrey Churbanov

Differential Revision: http://reviews.llvm.org/D21353


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@272945 91177308-0d34-0410-b5e6-96231b3b80d8