[OPENMP] Codegen for "omp flush" directive.

For each "omp flush" directive a call to "void kmpc_flush(ident_t *, ...)" function is generated.
Directive "omp flush" may have an associated list of variables to flush, but currently runtime function ignores them. So the patch generates just "call kmpc_flush(ident_t *<loc>, i32 0)".
Differential Revision: http://reviews.llvm.org/D6292

llvm-svn: 222409