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

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

if (@__kmpc_single()) {
  <init for firstprivates>
  @__kmpc_end_single();
}
@__kmpc_cancel_barrier(); // To avoid data race in firstprivate init
Differential Revision: http://reviews.llvm.org/D9223

llvm-svn: 235694