[AST] Give `CharUnits::operator%` a consistent type. NFC (#160781)

Update the `operator%` overload that accepts `CharUnits` to return
`CharUnits` to match the other `operator%`. This is more logical than
returning an `int64` and cleans up users that want to continue to do
math with the result.

Many users of this were explicitly comparing against 0. I considered
updating these to compare against `CharUnits::Zero` or even introducing
an `explicit operator bool()`, but they all feel clearer if we update
them to use the existing `isMultipleOf()` function instead.

0 files changed, 0 insertions, 0 deletions