Port from QStringViewLiteral to u""

Now that all our supported compilers know char16_t, we no longer need
QStringViewLiteral, whose only purpose in life was to turn u"" into
L"" for MSVC < 2015.

Change-Id: I1f5f1cb31c8e8278189f7634850b209b46127ce5
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>