[Reg 6.7.2->6.8.0b3] QByteArray::lastIndexOf fails to find char

When testing Qt 6.8.0 Beta 3 an issue regarding QByteArray::lastIndexOf() popped up.

This test snippet runs successfully with Qt 6.7.2 but fails with Qt 6.8.0 Beta 3. lastIndexOf returns -1 here as it fails to detect the casted char.

const auto a = QByteArray::fromHex("800080");
QCOMPARE(a.lastIndexOf(static_cast<char>(0x80)), 2);