c++: Ensure ANNOTATE_EXPRs remain outermost expressions in conditions [PR116140]

c++: Ensure ANNOTATE_EXPRs remain outermost expressions in conditions [PR116140]

For the testcase added with this patch, we would end up losing the:

  #pragma GCC unroll 4

and emitting "warning: ignoring loop annotation".  That warning comes
from tree-cfg.cc:replace_loop_annotate, and means that we failed to
process the ANNOTATE_EXPR in tree-cfg.cc:replace_loop_annotate_in_block.
That function walks backwards over the GIMPLE in an exiting BB for a
loop, skipping over the final gcond, and looks for any ANNOTATE_EXPRS
immediately preceding the gcond.

The function documents the following pre-condition:

   /* [...] We assume that the annotations come immediately before the
      condition in BB, if any.  */

now looking at the exiting BB of the loop, we have:

  <bb 8> :
  D.4524 = .ANNOTATE (iftmp.1, 1, 4);
  retval.0 = D.4524;
  if (retval.0 != 0)
    goto <bb 3>; [INV]
  else
    goto <bb 9>; [INV]

and crucially there is an intervening assignment between the gcond and
the preceding .ANNOTATE ifn call.  To see where this comes from, we can
look to the IR given by -fdump-tree-original:

  if (<<cleanup_point ANNOTATE_EXPR <first != last && !use_find(short
    int*)::<lambda(short int)>::operator() (&pred, *first), unroll 4>>>)
    goto <D.4518>;
  else
    goto <D.4516>;

here the problem is that we've wrapped a CLEANUP_POINT_EXPR around the
ANNOTATE_EXPR, meaning the ANNOTATE_EXPR is no longer the outermost
expression in the condition.

The CLEANUP_POINT_EXPR gets added by the following call chain:

finish_while_stmt_cond
 -> maybe_convert_cond
 -> condition_conversion
 -> fold_build_cleanup_point_expr

this patch chooses to fix the issue by first introducing a new helper
class (annotate_saver) to save and restore outer chains of
ANNOTATE_EXPRs and then using it in maybe_convert_cond.

With this patch, we don't get any such warning and the loop gets unrolled as
expected at -O2.

gcc/cp/ChangeLog:

	PR libstdc++/116140
	* semantics.cc (anotate_saver): New. Use it ...
	(maybe_convert_cond): ... here, to ensure any ANNOTATE_EXPRs
	remain the outermost expression(s) of the condition.

gcc/testsuite/ChangeLog:

	PR libstdc++/116140
	* g++.dg/ext/pragma-unroll-lambda.C: New test.