QChar: prepare relational operators for constexpr'ification

Make sure the relational operators are in a constexpr'able form
by removing the use of the const/non-const-overloaded unicode()
function, which in the relational operators is resolved to the
non-const version, which isn't constexpr'able.

Replaced with direct member access for op== and op< (required
making them friends) and reformulating the other operators in
terms of these two.

Since I managed to introduce a bug while doing this change,
add a simple test for QChar operators, too.

Change-Id: I69f3da849e71abc2a17152f797694950914adebc
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
QChar: mark some more functions constexpr

Change-Id: Iff82417b6da751278066cd325c4768cd24d56870
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
QChar: mark even more functions constexpr

These differ from the functions already marked constexpr in that
they contain subexpressions that are not constexpr. This is fine
according to the standard as long as there's _one_ set of arguments
that yield a Core Constant Expression (paraphrased from N3797
[dcl.constexpr]/5, C++11 contains similar wording), but maybe not
for some compilers, so collect these in a separate change.

Change-Id: If834232fd83059fce7c44d16b71e7d08ef79de65
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>