An unboxed tuple such as

    (# | b | | | | | #)

Ends up in the parser via `tup_exprs` as

    Sum 2 7 lexp

where `lexp` is a `LHsExpr`

From an API annotation perspective, the 5 `AnnVbar`s after the `b` were attached
to `lexp`, but the leading `AnnVbar`s did not have a home.

This patch attaches them all to the the parent tuple expression. The first (alt
- 1) of them come before `lexp`, and the remaining (arity - alt) come after.

Test Plan: ./validate

Reviewers: osa1, austin, bgamari

Subscribers: thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2968

GHC Trac Issues: #12417