Don't lose Unicode data when outputting wxString to std::ostream

Fall back to UTF-8 rather than not outputting anything at all if the string is
not representable in the current locale encoding.

Even if we did try to handle this error by setting failbit, chances of anybody
checking for it (especially on e.g. std::cout) were very low and the only
possible workaround in practice would have been attempting to output the
string in UTF-8 anyhow, so just do it ourselves.

See #17358.

src/common/string.cpp

@@ -191,13 +191,7 @@ static wxStrCacheStatsDumper s_showCacheStats;
 wxSTD ostream& operator<<(wxSTD ostream& os, const wxCStrData& str)
 {
 #if wxUSE_UNICODE && !wxUSE_UNICODE_UTF8
-    const wxScopedCharBuffer buf(str.AsCharBuf());
-    if ( !buf )
-        os.clear(wxSTD ios_base::failbit);
-    else
-        os << buf.data();
-
-    return os;
+    return os << wxConvWhateverWorks.cWX2MB(str);
 #else
     return os << str.AsInternal();
 #endif