[OPENMP 4.0] Codegen for 'cancellation point' directive.

The next code is generated for this construct:
```
if (__kmpc_cancellationpoint(ident_t *loc, kmp_int32 global_tid, kmp_int32 cncl_kind) != 0)
  <exit from outer innermost construct>;
```

llvm-svn: 241239