Port from qAsConst() to std::as_const()

We've been requiring C++17 since Qt 6.0, and our qAsConst use finally
starts to bother us (QTBUG-99313), so time to port away from it
now.

Since qAsConst has exactly the same semantics as std::as_const (down
to rvalue treatment, constexpr'ness and noexcept'ness), there's really
nothing more to it than a global search-and-replace.

Task-number: QTBUG-99313
Change-Id: I88edd91395849574436299b8badda21bb93bea39
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cppeditor/cppcompletion_test.cpp

@@ -258,7 +258,7 @@ void CompletionTest::testCompletionTemplateFunction()
 
     QStringList actualCompletions = test.getCompletions();
     QString errorPattern(QLatin1String("Completion not found: %1"));
-    for (const QString &completion : qAsConst(expectedCompletions))  {
+    for (const QString &completion : std::as_const(expectedCompletions))  {
         QByteArray errorMessage = errorPattern.arg(completion).toUtf8();
         QVERIFY2(actualCompletions.contains(completion), errorMessage.data());
     }