Fix ICE (issue 8262).

FIxes: https://issues.dlang.org/show_bug.cgi?id=8262

This PR is still incomplete, as it still doesn't quite implement the `alias this` in the expected manner (the output of the test program in the bug is `S(0)` as opposed to `0`, which would be the case if the extra level of indirection via `Seq!s` was removed), and the error appears to be swallowed. At least the compiler won't segfault anymore, but probably this isn't the correct fix for the problem.
Haha who knows.  It's the megatest.