Checked Span: Move `IsExtentInBounds()` out

Including the headers for `CheckedNumeric` (and maybe `StrictNumeric`)
causes the compile-size bot to report ~2GiB of bloat.

Move these out-of-line. `IsExtentInBounds()` currently has no callers
(and, when we think about it, perhaps it may never gain any, unless
Project Fortify changes quite a bit), so the change is self-contained.

Bug: 501113358
Change-Id: I9911d0fb7fe94ebb83be5707fdf23b31e8ecf340
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7762152
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1615924}
NOKEYCHECK=True
GitOrigin-RevId: 7518d3ca0b96c500a1cf5be523f156c22282ce4b