[clang][OpenMP] OMPFlushClause is synthetic, no such clause exists

Summary:
As discussed in https://reviews.llvm.org/D57112#inline-506781,
'flush' clause does not exist in the OpenMP spec, it can not be
specified, and `OMPFlushClause` class is just a helper class.

Therefore `OPENMP_CLAUSE()` in `clang/Basic/OpenMPKinds.def`
should not contain 'flush' "clause".

I have simply removed the `OPENMP_CLAUSE(flush, OMPFlushClause)`
from `clang/Basic/OpenMPKinds.def`, grepped for `OPENMP_CLAUSE`
and added `OPENMP_CLAUSE(flush, OMPFlushClause)` back to the **every**
place where `OPENMP_CLAUSE` is defined and `clang/Basic/OpenMPKinds.def`
is then included.

So as-is, this patch is a NFC. Possibly, some of these
`OPENMP_CLAUSE(flush, OMPFlushClause)` should be dropped,
i don't really know.

Test plan: `ninja check-clang`

Reviewers: ABataev

Reviewed By: ABataev

Subscribers: guansong, arphaman, cfe-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D57280

llvm-svn: 352390