[PATCH] Fix PR c++/36408, second attempt.

[Jason Merrill]

Dodji Seketeli wrote:
> The idea of this patch is to reduce empty statement-expressions to 
> void_zero_node, early in the process i.e right at parsing time rather 
> than trying to fix things too late, at template instantiation time.

This seems fine, but doesn't fix the related bug we were talking about 
with folding non-empty statement-expressions; a fix for that bug ought 
to fix this one as well.

Jason