[OPENMP] Additional processing of 'omp atomic read' directive.

[OPENMP] Additional processing of 'omp atomic read' directive.
According to OpenMP standard, Section 2.12.6, atomic Construct, '#pragma omp atomic read' is allowed to be used only for expression statements of form 'v = x;', where x and v (as applicable) are both l-value expressions with scalar type. Patch adds checks for it.

llvm-svn: 222231