QStringConverter: port (name, Flags) ctor to QAnyStringView

These kinds of lookup functions are no-brainers for QAnyStringView.

This is required to replace the QStringEncoder/Decoder constructors
from QString, newly added for 6.8, and found in API review to be
better written using QAnyStringView.

Because this is API-Review-relevant, keep the implementation
pessimized. It's not worse than the code that it replaces (toLatin1()
on an unshared QString performs in-place conversion), and can be
improved later.

Task-number: QTBUG-126108
Change-Id: I4a0d1840d4754670b2e2f20d768ed98f7e1937d5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit f6c11ac4f20a16d0b2113014e2dac63b95d946ae)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/compat/removed_api.cpp

@@ -1148,6 +1148,10 @@ bool QtPrivate::ResultIteratorBase::operator!=(const QtPrivate::ResultIteratorBa
 
 #include "qstringconverter.h"
 
+QStringConverter::QStringConverter(const char *name, Flags f)
+    : QStringConverter(QAnyStringView{name}, f)
+{}
+
 auto QStringConverter::encodingForName(const char *name) noexcept -> std::optional<Encoding>
 {
     return encodingForName(QAnyStringView{name});

src/corelib/text/qstringconverter.cpp

@@ -2146,6 +2146,10 @@ struct QStringConverterICU : QStringConverter
         return conv;
     }
 
+    static const QStringConverter::Interface *
+    make_icu_converter(QStringConverterBase::State *state, QAnyStringView name)
+    { return make_icu_converter(state, name.toString().toLatin1().constData()); } // ### optimize
+
     static const QStringConverter::Interface *make_icu_converter(
             QStringConverterBase::State *state,
             const char *name)
@@ -2183,7 +2187,7 @@ struct QStringConverterICU : QStringConverter
 /*!
     \internal
 */
-QStringConverter::QStringConverter(const char *name, Flags f)
+QStringConverter::QStringConverter(QAnyStringView name, Flags f)
     : iface(nullptr), state(f)
 {
     auto e = encodingForName(name);

src/corelib/text/qstringconverter_base.h

@@ -138,7 +138,10 @@ protected:
     constexpr explicit QStringConverter(const Interface *i) noexcept
         : iface(i)
     {}
+#if QT_CORE_REMOVED_SINCE(6, 8)
     Q_CORE_EXPORT explicit QStringConverter(const char *name, Flags f);
+#endif
+    Q_CORE_EXPORT explicit QStringConverter(QAnyStringView name, Flags f);
 
 
     ~QStringConverter() = default;