Add an llvm::OStream workalike class for use with Logger::cout(), with the

crucial difference being special handling of `llvm::Type`s so they get printed
by name rather than printing their full representation (which can be positively
*huge*).

This allows re-enabling some logger calls that were disabled due to extreme
verbosity.

gen/abi.cpp

@@ -385,10 +385,8 @@ struct IntrinsicABI : TargetABI
             if (ty->ty == Tstruct)
                 fixup(arg);
 
-#if 0
             if (Logger::enabled())
                 Logger::cout() << "New arg type: " << *arg.ltype << '\n';
-#endif
         }
     }
 };