Use wxUniCharRef rather than "auto&&" to iterate over wxString

As explained in 0a5be41f8a (Avoid using uninitialized static wxString in
wxOSX code, 2021-04-04), the usual "auto&" can't be used to iterate over
wxString and modify its contents because of proxy-like nature of
wxUniCharRef returned by wxString iterators.

But using "auto&&" as that commit did wasn't especially clear (even with
the explanations in the commit message) and gave -Wrange-loop-analysis
when using clang.

So replace it with "wxUniCharRef", which should be both more clear (it
is a value, which is still a bit confusing, but its name hopefully
indicates that it's also some kind of a reference) and warning-free.

src/osx/core/cfstring.cpp

@@ -28,7 +28,7 @@
 wxString wxMacConvertNewlines13To10(const wxString& data)
 {
     wxString string(data);
-    for (auto&& c: string)
+    for (wxUniCharRef c: string)
     {
         if (c == '\r')
             c = '\n';
@@ -39,7 +39,7 @@ wxString wxMacConvertNewlines13To10(const wxString& data)
 wxString wxMacConvertNewlines10To13(const wxString& data)
 {
     wxString string(data);
-    for (auto&& c: string)
+    for (wxUniCharRef c: string)
     {
         if (c == '\n')
             c = '\r';