E0004 bonus update: spans can't cross nodes

[sophiajt]

From #35485

After talking with @arielb1 (and seeing @nrc's comments) - let's actually change the span to be one we can do safely:

From:

match x {
^^^^^^^ pattern `HastaLaVistaBaby` not covered

To:

match x {
      ^ pattern `HastaLaVistaBaby` not covered

CC: @KiChjang

[KiChjang]

Okay, this should be a simple fix, I'm taking it.

[arielb1]

I have a mega-pr #35308 that includes it.

[arielb1]

Still, I find spanning the scrutinee is a bit odd. But spanning the whole match isn't so pretty, so...

[KiChjang]

Commented on your PR.