[OpenMP] diagnose assign to firstprivate const, patch by Joel E. Denny

Summary:
[OpenMP] diagnose assign to firstprivate const

Clang does not diagnose assignments to const variables declared
firstprivate.  Furthermore, codegen is broken such that, at run time,
such assignments simply have no effect.  For example, the following
prints 0 not 1:

int main() {
  const int i = 0;
  #pragma omp parallel firstprivate(i)
  { i=1; printf("%d\n", i); }
  return 0;
}

This commit makes these assignments a compile error, which is
consistent with other OpenMP compilers I've tried (pgcc 17.4-0, gcc
6.3.0).

Reviewers: ABataev

Reviewed By: ABataev

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D39859

llvm-svn: 317891