QVarLengthArray: clear() is not resize(0)

The latter needs the value_type to be default-constructible, which
shouldn't be required to clear() a container.

Use std::destroy_n() instead.

[ChangeLog][QtCore][QVarLengthArray] clear() no longer requires the
value_type to be default-constructible.

Change-Id: I806de8f3826b50c0bd38156892c3afeb15f13ac9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit fbfee2d7c59a7c6cd17ae7a3f63f983b9f3316f5)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/tools/qvarlengtharray.h

@@ -172,6 +172,13 @@ public:
     template <typename Predicate>
     qsizetype removeIf(Predicate pred);
 
+    void clear()
+    {
+        if constexpr (QTypeInfo<T>::isComplex)
+            std::destroy_n(data(), size());
+        s = 0;
+    }
+
     iterator erase(const_iterator begin, const_iterator end);
     iterator erase(const_iterator pos) { return erase(pos, pos + 1); }
 
@@ -400,7 +407,10 @@ public:
 #endif
     void resize(qsizetype sz, const T &v)
     { Base::resize_impl(Prealloc, this->array, sz, v); }
+    using Base::clear;
+#ifdef Q_QDOC
     inline void clear() { resize(0); }
+#endif
     void squeeze() { reallocate(size(), size()); }
 
     using Base::capacity;