Use StringRef for Opt's optNames

Removes another ~70 allocations.

src/catch2/internal/catch_clara.cpp

@@ -25,13 +25,16 @@ namespace {
             ;
     }
 
-    std::string normaliseOpt( std::string const& optName ) {
-#ifdef CATCH_PLATFORM_WINDOWS
-        if ( optName[0] == '/' )
-            return "-" + optName.substr( 1 );
-        else
+    Catch::StringRef normaliseOpt( Catch::StringRef optName ) {
+        if ( optName[0] == '-'
+#if defined(CATCH_PLATFORM_WINDOWS)
+             || optName[0] == '/'
 #endif
-            return optName;
+        ) {
+            return optName.substr( 1, optName.size() );
+        }
+
+        return optName;
     }
 
 } // namespace