[OPENMP] Initial codegen for 'omp sections' and 'omp section' directives.

If only one section is found in the sections region, it is emitted just like single region.
Otherwise it is emitted as a static non-chunked loop.

#pragma omp sections
{
#pragma omp section
  {1}
  ...
  #pragma omp section
  {n}
}
is translated to something like

i32 <iter_var>
i32 <last_iter> = 0
i32 <lower_bound> = 0
i32 <upper_bound> = n-1
i32 <stride> = 1
call void @__kmpc_for_static_init_4(<loc>, i32 <gtid>, i32 34/*static non-chunked*/, i32* <last_iter>, i32* <lower_bound>, i32* <upper_bound>, i32* <stride>, i32 1/*increment always 1*/, i32 1/*chunk always 1*/)
<upper_bound> = min(<upper_bound>, n-1)
<iter_var> = <lb>
check:
br <iter_var> <= <upper_bound>, label cont, label exit
continue:
switch (IV) {
  case 0:
  {1};
  break;
  ...
  case <NumSection> - 1:
  {n};
  break;
  }
  ++<iter_var>
  br label check
  exit:
  call void @__kmpc_for_static_fini(<loc>, i32 <gtid>)
Differential Revision: http://reviews.llvm.org/D8244

llvm-svn: 232021