[OPENMP-SIMD] Fix PR37536: Fix definition of _OPENMP macro.

if `-fopenmp-simd` is specified alone, `_OPENMP` macro should not be
  defined. If `-fopenmp-simd` is specified along with the `-fopenmp`,
  `_OPENMP` macro should be defined with the value `201511`.

llvm-svn: 332852