Move QTypeInfo<std::pair> from qpair.h to qtypeinfo.h to avoid ODR violation

First off, this doesn't cost us anything, because std::pair is defined
in <utility>, which qglobal.h unconditionally includes in C++ TUs.

More importantly, it prevents ODR violations: when a TU includes only
qtypeinfo.h, it will find std::pair<int, int> to be of Q_COMPLEX_TYPE,
in constrast with a TU which includes qpair.h, which will find it to
be of Q_PRIMITIVE_TYPE instead.

[ChangeLog][QtCore][QTypeInfo] The QTypeInfo for std::pair/QPair will
now be correct even if qpair.h hasn't been included, fixing an
One-Definition-Rule (ODR) violation.

Pick-to: 6.5 6.4 6.2
Change-Id: I51f579c123183af25aac9f0ffcf077f752848fb1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qtypeinfo.h

@@ -94,6 +94,12 @@ public:
     static constexpr bool isValueInitializationBitwiseZero = false;
 };
 
+// QTypeInfo for std::pair:
+//   std::pair is spec'ed to be struct { T1 first; T2 second; }, so, unlike tuple<>,
+//   we _can_ specialize QTypeInfo for pair<>:
+template <class T1, class T2>
+class QTypeInfo<std::pair<T1, T2>> : public QTypeInfoMerger<std::pair<T1, T2>, T1, T2> {};
+
 #define Q_DECLARE_MOVABLE_CONTAINER(CONTAINER) \
 template <typename ...T> \
 class QTypeInfo<CONTAINER<T...>> \

src/corelib/tools/qpair.h

@@ -22,9 +22,6 @@ constexpr decltype(auto) qMakePair(T1 &&value1, T2 &&value2)
     return std::make_pair(std::forward<T1>(value1), std::forward<T2>(value2));
 }
 
-template<class T1, class T2>
-class QTypeInfo<std::pair<T1, T2>> : public QTypeInfoMerger<std::pair<T1, T2>, T1, T2> {};
-
 QT_END_NAMESPACE
 
 #endif // QPAIR_H