[OPENMP] Codegen for 'omp barrier' directive.

Adds generation of call to "i32 kmpc_cancel_barrier(ident_t *, i32)" libcall for explicitly specified barriers (OMP_IDENT_BARRIER_EXPL flag is added to "flags" field of "ident_t" structure).
Also this patch replaces all calls to "kmpc_barrier" function by calls of "__kmpc_cancel_barrier" function which provides additional functionality for OpenMP 4.0.
Also, library specific enum OpenMPLocationFlags moved to private section of CGOpenMPRuntime class to make it more independent from library implementation.
Differential Revision: http://reviews.llvm.org/D6447

llvm-svn: 223444