QXmlStreamReader: don't convert input

When we ported QXmlStreamReader to QAnyStringView, the ctor was left to do, essentially, d->dataBuffer = data.toString().toUtf8(). In particular, the L1 case is first converted to U16 and then to U8. The reason this is done so is that addData() can assume that dataBuffer is in U8 encoding. But then addData()'s re-setting of the decoder is problematic when the dataBuffer still contains unparsed data (can it?).

The idea of the QAnyStringView ctor is to not convert, but adjust the decoder accordingly.