QVarLengthArray: simplify copy ctor implementation

std::tie(a, s, ptr) = {Prealloc, 0, (T*)array} is exactly the effects
of the default ctor, so just delegate to it, before calling
copyAppend().

Also clean up the copy ctor declaration (superfluous inline keyword,
and template arguments).

Change-Id: Iefc758a47996382fee10b1e081e44f008b146287
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qvarlengtharray.h

@@ -67,8 +67,8 @@ public:
 
     inline explicit QVarLengthArray(qsizetype size);
 
-    inline QVarLengthArray(const QVarLengthArray<T, Prealloc> &other)
-        : a(Prealloc), s(0), ptr(reinterpret_cast<T *>(array))
+    QVarLengthArray(const QVarLengthArray &other)
+        : QVarLengthArray{}
     {
         append(other.constData(), other.size());
     }