Avoid using uninitialized static wxString in wxOSX code

Global "sCR" and "sLF" string objects could have been not yet
initialized when wxMacConvertNewlines{13To10,10To13}() were called as
these functions can be (implicitly) used when initializing other static
objects.

Fix the problem by avoiding the use of these objects, as well of
wxString::Replace(), entirely and just iterating over the string
directly using a simple, and more efficient, for loop.

Note the use of "auto&&": a more usual "auto&" can't be used with
temporary wxUniCharRef created by dereferencing wxString iterators.
Using just "auto" would have actually worked too, but modifying a value
via a copy would seem surprising, so use "auto&&" as a hopefully
slightly more clear alternative.

Closes #19110.

src/osx/core/cfstring.cpp

@@ -25,20 +25,25 @@
 #include <CoreFoundation/CoreFoundation.h>
 #include <CoreFoundation/CFData.h>
 
-const wxString sCR((wxChar)13);
-const wxString sLF((wxChar)10);
-
 wxString wxMacConvertNewlines13To10(const wxString& data)
 {
     wxString string(data);
-    string.Replace(sCR, sLF);
+    for (auto&& c: string)
+    {
+        if (c == '\r')
+            c = '\n';
+    }
     return string;
 }
 
 wxString wxMacConvertNewlines10To13(const wxString& data)
 {
     wxString string(data);
-    string.Replace(sLF, sCR);
+    for (auto&& c: string)
+    {
+        if (c == '\n')
+            c = '\r';
+    }
     return string;
 }