Fix using std::reverse() with wxString iterators in a proper way

The solution with specializing std::iter_swap() for wxString::iterator
was not conforming as the iterator was still not swappable, as it is
required to be.

Fix this by providing std::swap() overload for wxString::iterator, which
is correct and even simpler.

This allows std::reverse(s.begin(), s.end()) work with clang too and
incidentally avoids warnings about the code relying on non-conforming
extensions with MSVS 2017 which were due to the fact that iter_swap()
workaround wasn't enabled for it, while the new swap() overload is.

include/wx/unichar.h

@@ -360,6 +360,23 @@ void swap<wxUniCharRef>(wxUniCharRef& lhs, wxUniCharRef& rhs)
 
 } // namespace std
 
+#if __cplusplus >= 201103L || wxCHECK_VISUALC_VERSION(10)
+
+// For std::iter_swap() to work with wxString::iterator, which uses
+// wxUniCharRef as its reference type, we need to ensure that swap() works with
+// wxUniCharRef objects by defining this overload.
+//
+// See https://bugs.llvm.org/show_bug.cgi?id=28559#c9
+inline
+void swap(wxUniCharRef&& lhs, wxUniCharRef&& rhs)
+{
+    wxUniChar tmp = lhs;
+    lhs = rhs;
+    rhs = tmp;
+}
+
+#endif // C++11
+
 
 // Comparison operators for the case when wxUniChar(Ref) is the second operand
 // implemented in terms of member comparison functions