Ben Gamari authored Jun 04, 2019 and Marge Bot committed Jun 13, 2019

Previously the presence of source notes could hide nested applications
of `unpackFoldrCString#` from our constant folding logic. For instance,
consider the expression:

```haskell
unpackFoldrCString# "foo" c (unpackFoldrCString# "baz" c n)
```

Specifically, ticks appearing in two places can defeat the rule:

  a. Surrounding the inner application of `unpackFoldrCString#`
  b. Surrounding the fold function, `c`

The latter caused the `str_rules` testcase to fail when `base` was built
with `-g3`.

Fixes #16740.