[OPENMP] Codegen for 'omp master' directive

Patch adds 2 library functions to OpenMPRuntime class - int32 kmpc_master(ident_t *, int32 gtid) and void kmpc_end_master(ident_t *, int32 gtid);
For 'omp master' directive the next code is generated:

if (__kmpc_master(loc, gtid)) {
    <Associated structured block>;
      __kmpc_end_master(log, gtid);
}

Differential Revision: http://reviews.llvm.org/D6473

llvm-svn: 223342