QVarLengthArray: simplify default ctor

Instead of calling the QVLA(qsizetype) ctor and letting the optimizer
and inliner have a field day on them, simply set the trivial values
for a, s, and ptr manually.

Mark it as noexcept.

Task-number: QTBUG-84785
Change-Id: I011826eacdaf63a4c37b21465e5fe4a6e70a1ab7
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/tools/qvarlengtharray.h

@@ -63,7 +63,10 @@ class QVarLengthArray
     static_assert(std::is_nothrow_destructible_v<T>, "Types with throwing destructors are not supported in Qt containers.");
 
 public:
-    QVarLengthArray() : QVarLengthArray(0) {}
+    QVarLengthArray() noexcept
+        : a{Prealloc}, s{0}, ptr{reinterpret_cast<T *>(array)}
+    {
+    }
 
     inline explicit QVarLengthArray(qsizetype size);