[OPENMP] Codegen for 'private' clause in 'single' directive.

Emit the following code for 'single' directive with 'private' clause:

if (@__kmpc_single()) {
  <default init for privates>
  @__kmpc_end_single();
}
Differential Revision: http://reviews.llvm.org/D9238

llvm-svn: 235832