Fix build with MSVC 2015 Update 2 if constexpr is enabled

This compiler seems to require explicit initialization of all member
variables in a constexpr constructor, even if they have an implicit
default constructor of their own. We probably fixed the rest of Qt a
couple of years ago, but not these two places because they were arrays
and those require the C++11 syntax for uniform initialization.

All compilers that support constexpr do support uniform initialization.
MSVC 2015 fixed our issues with it on the same update.

Change-Id: Ibc1eb23e3ae093f5c6928ded3a041be35eb9baae
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qfreelist_p.h

@@ -207,7 +207,11 @@ public:
 
 template <typename T, typename ConstantsType>
 Q_DECL_CONSTEXPR inline QFreeList<T, ConstantsType>::QFreeList()
-    : _next(ConstantsType::InitialNextValue)
+    :
+#if defined(Q_COMPILER_CONSTEXPR)
+      _v{}, // uniform initialization required
+#endif
+      _next(ConstantsType::InitialNextValue)
 { }
 
 template <typename T, typename ConstantsType>

src/widgets/widgets/qcalendarwidget.cpp

@@ -832,7 +832,12 @@ class StaticDayOfWeekAssociativeArray {
     static Q_DECL_CONSTEXPR int day2idx(Qt::DayOfWeek day) Q_DECL_NOTHROW { return int(day) - 1; } // alt: day % 7
 public:
     Q_DECL_CONSTEXPR StaticDayOfWeekAssociativeArray() Q_DECL_NOEXCEPT_EXPR(noexcept(T()))
-        : contained(), data() {}
+#ifdef Q_COMPILER_CONSTEXPR
+        : contained{}, data{}   // arrays require uniform initialization
+#else
+        : contained(), data()
+#endif
+    {}
 
     Q_DECL_CONSTEXPR bool contains(Qt::DayOfWeek day) const Q_DECL_NOTHROW { return contained[day2idx(day)]; }
     Q_DECL_CONSTEXPR const T &value(Qt::DayOfWeek day) const Q_DECL_NOTHROW { return data[day2idx(day)]; }