Clean up extraneous copies in Messages

This removes 109 allocations from running `tests/SelfTest`
(437057 -> 436948).

src/catch2/interfaces/catch_interfaces_reporter.cpp

@@ -60,10 +60,9 @@ namespace Catch {
             // Copy message into messages list.
             // !TBD This should have been done earlier, somewhere
             MessageBuilder builder( assertionResult.getTestMacroName(), assertionResult.getSourceInfo(), assertionResult.getResultType() );
-            builder << assertionResult.getMessage();
-            builder.m_info.message = builder.m_stream.str();
+            builder.m_info.message = static_cast<std::string>(assertionResult.getMessage());
 
-            infoMessages.push_back( builder.m_info );
+            infoMessages.push_back( CATCH_MOVE(builder.m_info) );
         }
     }