QString: get rid of public qStringComparisonHelper()

QString: get rid of public qStringComparisonHelper()

Unify all it's overloads into a single private helper functionand use this new helper
where possible - so we could optimize all those operators in one step some later
(this also fixes `QBytArray("a\0b") < "a"` didn't respect the \0 while operator==
handles nul(s) correctly);
Add operators <,>,<=,>=(const char*) to QStringRef so that they doesn't create a temporary QString object;
Add missing QT_NO_CAST_FROM_ASCII

Change-Id: I8b6562a92fdb96e67aadee181f23f823d206f5fd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>