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_capture_matchers.cpp

@@ -15,7 +15,7 @@ namespace Catch {
     // This is the general overload that takes a any string matcher
     // There is another overload, in catch_assertionhandler.h/.cpp, that only takes a string and infers
     // the Equals matcher (so the header does not mention matchers)
-    void handleExceptionMatchExpr( AssertionHandler& handler, StringMatcher const& matcher, StringRef matcherString  ) {
+    void handleExceptionMatchExpr( AssertionHandler& handler, StringMatcher const& matcher, StringRef const& matcherString  ) {
         std::string exceptionMessage = Catch::translateActiveException();
         MatchExpr<std::string, StringMatcher const&> expr( exceptionMessage, matcher, matcherString );
         handler.handleExpr( expr );