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.cpp

@@ -52,7 +52,7 @@ namespace Catch {
     }
 
     AssertionHandler::AssertionHandler
-        (   StringRef macroName,
+        (   StringRef const& macroName,
             SourceLineInfo const& lineInfo,
             StringRef capturedExpression,
             ResultDisposition::Flags resultDisposition )
@@ -109,7 +109,7 @@ namespace Catch {
 
     // This is the overload that takes a string and infers the Equals matcher from it
     // The more general overload, that takes any string matcher, is in catch_capture_matchers.cpp
-    void handleExceptionMatchExpr( AssertionHandler& handler, std::string const& str, StringRef matcherString  ) {
+    void handleExceptionMatchExpr( AssertionHandler& handler, std::string const& str, StringRef const& matcherString  ) {
         handleExceptionMatchExpr( handler, Matchers::Equals( str ), matcherString );
     }