Avoid copying StringRef

In theory the copy is cheap (couple of pointers change), but tests
are usually compiled in Debug mode/with minimal optimizations, which
means that most users will still have to pay the cost for those
function calls.

include/internal/catch_assertionhandler.h

@@ -49,7 +49,7 @@ namespace Catch {
 
     public:
         AssertionHandler
-            (   StringRef macroName,
+            (   StringRef const& macroName,
                 SourceLineInfo const& lineInfo,
                 StringRef capturedExpression,
                 ResultDisposition::Flags resultDisposition );
@@ -81,7 +81,7 @@ namespace Catch {
         auto allowThrows() const -> bool;
     };
 
-    void handleExceptionMatchExpr( AssertionHandler& handler, std::string const& str, StringRef matcherString );
+    void handleExceptionMatchExpr( AssertionHandler& handler, std::string const& str, StringRef const& matcherString );
 
 } // namespace Catch