Use StringRef through XmlEncode

This introduces a potential lifetime risk when using the API, but
the intended way to use the `XmlEncode` class is to use it directly,
e.g. `out << XmlEncode(some-text-argument)`, not to store it around.

The benefit is that we avoid allocations for strings that do not fit
into SSO for given platform.

src/catch2/internal/catch_xmlwriter.cpp

@@ -77,7 +77,7 @@ namespace {
     }
 
 
-    XmlEncode::XmlEncode( std::string const& str, ForWhat forWhat )
+    XmlEncode::XmlEncode( StringRef str, ForWhat forWhat )
     :   m_str( str ),
         m_forWhat( forWhat )
     {}