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

[OPENMP] Additional processing of 'omp atomic write' directive.
According to OpenMP standard, Section 2.12.6, atomic Construct, '#pragma omp atomic write' is allowed to be used only for expression statements of form 'x = expr;', where x is a lvalue expression and expr is an expression with scalar type. Patch adds checks for it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222913 91177308-0d34-0410-b5e6-96231b3b80d8