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.

tests/strings/stdstrings.cpp

@@ -626,13 +626,7 @@ void StdStringTestCase::StdConversion()
 
 void StdStringTestCase::StdAlgo()
 {
-    // Unfortunately this currently doesn't work with libc++ in C++11 mode, see
-    // comment near iter_swap() definition in wx/string.h.
-#if __cplusplus < 201103L || !defined(_LIBCPP_VERSION)
     wxString s("AB");
     std::reverse(s.begin(), s.end());
     CPPUNIT_ASSERT_EQUAL( "BA", s );
-#else
-    wxLogWarning("Skipping std::reverse() test broken with C++11/libc++");
-#endif
 }