[OPENMP] Create non-const ident_t objects.

Currently ident_t objects are created const when debug info is not
enabled, but the libittnotify libray in the OpenMP runtime writes to
the reserved_2 field (See __kmp_itt_region_forking in
openmp/runtime/src/kmp_itt.inl).  Now create ident_t objects non-const.

Differential Revision: https://reviews.llvm.org/D51331

llvm-svn: 340934