The implementation now uses the relaxed-constexpr
qCountTrailingZeroBits() function from QtAlgorithms, making the
QSizePolicy(Policy, Policy, ControlType) constructor constexpr on
C++14 compilers. The explicit check for DefaultType remains to keep
the constructor C++11-constexpr when called with just (Policy,
Policy).
Extend the constExpr tests a bit.
Change-Id: I59690f0921d9bdee08e3615d0d1f4a9b92870c32
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>