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).

This was previously reverted in 8e4a8677 for rejecting some code, but that
code was invalid and Clang was previously incorrectly accepting it.