PR c++/33799 - destroy return value if local cleanup throws.

This is a pretty rare situation since the C++11 change to make all
destructors default to noexcept, but it is still possible to define throwing
destructors, and if a destructor for a local variable throws during the
return, we've already constructed the return value, so now we need to
destroy it.  I handled this somewhat like the new-expression cleanup; as in
that case, this cleanup can't properly nest with the cleanups for local
variables, so I introduce a cleanup region around the whole function and a
flag variable to indicate whether the return value actually needs to be
destroyed.

Setting the flag requires giving a COMPOUND_EXPR as the operand of a
RETURN_EXPR, so I adjust gimplify_return_expr to handle that.

This doesn't currently work with deduced return type because we don't know
the type when we're deciding whether to introduce the cleanup region.

gcc/
	* gimplify.c (gimplify_return_expr): Handle COMPOUND_EXPR.
gcc/cp/
	* cp-tree.h (current_retval_sentinel): New macro.
	* decl.c (start_preparsed_function): Set up cleanup for retval.
	* typeck.c (check_return_expr): Set current_retval_sentinel.