Replace some QString::fromUtf16() with QStringView::toString()

The QStringView counterpart is somewhat faster because it doesn't go
through the UTF-16 codec in QUtf16::convertToUnicode(), which tries to
detect the BOM.

I've included QString::fromWCharArray in this because:
a) it's used extensively in Windows code
b) wide chars in memory probably don't have BOMs anyway

Change-Id: I01ec3c774d9943adb903fffd17b815be4d2ab8ba
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>

src/corelib/io/qdir.cpp

@@ -2353,7 +2353,7 @@ QString qt_normalizePathSegments(const QString &name, QDirPrivate::PathNormaliza
     // If path was not modified return the original value
     if (used == 0)
         return name;
-    return QString::fromUtf16(out + used, len - used);
+    return QStringView(out + used, len - used).toString();
 }
 
 static QString qt_cleanPath(const QString &path, bool *ok)

src/corelib/text/qregularexpression.cpp

@@ -1508,7 +1508,7 @@ QStringList QRegularExpression::namedCaptureGroups() const
                 reinterpret_cast<const char16_t *>(namedCapturingTable) + namedCapturingTableEntrySize * i;
 
         const int index = *currentNamedCapturingTableRow;
-        result[index] = QString::fromUtf16(currentNamedCapturingTableRow + 1);
+        result[index] = QStringView(currentNamedCapturingTableRow + 1).toString();
     }
 
     return result;

src/corelib/text/qstring.h

@@ -1307,8 +1307,11 @@ QT_WARNING_POP
 
 QString QString::fromWCharArray(const wchar_t *string, qsizetype size)
 {
-    return sizeof(wchar_t) == sizeof(QChar) ? fromUtf16(reinterpret_cast<const char16_t *>(string), size)
-                                            : fromUcs4(reinterpret_cast<const char32_t *>(string), size);
+    if constexpr (sizeof(wchar_t) == sizeof(QChar)) {
+        return QString(reinterpret_cast<const QChar *>(string), size);
+    } else {
+        return fromUcs4(reinterpret_cast<const char32_t *>(string), size);
+    }
 }
 
 constexpr QString::QString() noexcept {}

src/plugins/networkinformation/networkmanager/qnetworkmanagernetworkinformationbackend.cpp

@@ -98,8 +98,8 @@ bool isMeteredFromNMMetered(QNetworkManagerInterface::NMMetered metered)
 
 static QString backendName()
 {
-    return QString::fromUtf16(QNetworkInformationBackend::PluginNames
-                                      [QNetworkInformationBackend::PluginNamesLinuxIndex]);
+    return QStringView(QNetworkInformationBackend::PluginNames
+                       [QNetworkInformationBackend::PluginNamesLinuxIndex]).toString();
 }
 
 QString QNetworkManagerNetworkInformationBackend::name() const

src/plugins/platforms/xcb/qxcbmime.cpp

@@ -155,7 +155,7 @@ QVariant QXcbMime::mimeConvertToFormat(QXcbConnection *connection, xcb_atom_t a,
         const quint8 byte1 = data.at(1);
         if ((byte0 == 0xff && byte1 == 0xfe) || (byte0 == 0xfe && byte1 == 0xff)
             || (byte0 != 0 && byte1 == 0) || (byte0 == 0 && byte1 != 0)) {
-            const QString str = QString::fromUtf16(
+            const QStringView str(
                   reinterpret_cast<const char16_t *>(data.constData()), data.size() / 2);
             if (!str.isNull()) {
                 if (format == "text/uri-list"_L1) {
@@ -174,7 +174,7 @@ QVariant QXcbMime::mimeConvertToFormat(QXcbConnection *connection, xcb_atom_t a,
                         return list.constFirst();
                     return list;
                 } else {
-                    return str;
+                    return str.toString();
                 }
             }
         }