It turns out that `Convert` was recklessly leaving off
parentheses in two places:

* Negative numeric literals
* Patterns in lambda position

This patch fixes it by adding three new functions, `isCompoundHsLit`,
`isCompoundHsOverLit`, and `isCompoundPat`, and using them in the
right places in `Convert`. While I was in town, I also sprinkled
`isCompoundPat` among some `Pat`-constructing functions in `HsUtils`
to help avoid the likelihood of this problem happening in other
places. One of these places is in `TcGenDeriv`, and sprinkling
`isCompountPat` there fixes #14682

Test Plan: make test TEST="T14681 T14682"

Reviewers: alanz, goldfire, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14681, #14682

Differential Revision: https://phabricator.haskell.org/D4323

(cherry picked from commit 575c009d)