[OpenMP] Ignore parens in atomic capture

Clang misdiagnoses atomic captures cases that contains parens.
i.e.

  int v, int *p;
#pragma omp atomic capture
{ v = (*p); (*p)++; }

Patch by David S.

Differential Revision: https://reviews.llvm.org/D22487


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276167 91177308-0d34-0410-b5e6-96231b3b80d8