QStringRef can't reference QStrings outside the first 2Gi characters

No-one ported QStringRef to qsizetype when QString was ported away from int, so now we're left with a QStringRef that has int size and offset members, and thus can't reference all QStrings. The only thing we can do, at this point, is document it and Q_ASSERT() when trying to construct a QStringRef that would reference a portion of QString outside the first 2Gi characters.

Technically speaking, we need to widen the setters to accept qsizetype (to prevent narrowing at the call site), but reject offset/length pairs that are not confined to the first 2Gi of characters.