PR45083: Mark statement expressions as being dependent if they contain

dependent constructs.

We previously assumed they were neither value- nor
instantiation-dependent under any circumstances, which would lead to
crashes and other misbehavior.

This doesn't match GCC's behavior (where statement expressions appear to
be treated as value-dependent if they appear in a dependent context),
but seems to be the best thing we can do in the short term: it turns out
to be remarkably difficult for us to correctly determine whether we are
in a dependent context (and it's not even possible in some cases, such
as in a generic lambda where we might not have seen the 'auto' yet).