[OpenMP][OMPT] Fix unsafe initialization of ompt_data_t objects

Initializing an ompt_data_t object using the pointer union member is potentially
unsafe in 32-bit programs.  This change fixes the issue
by using the constant, ompt_data_none.

Patch by Hansang Bae

Differential Revision: https://reviews.llvm.org/D52046

llvm-svn: 343785