Use LLVM OStream wrapper instead of <iostream> in the logger.

llvm::OStream provides all std::ostream functionality (by holding a
std::ostream* internally), but
 * doesn't include <iostream>, avoiding per-file overhead.
 * allows the stream pointer to be null, and the (inlined) operators do nothing
   when that's the case. (This also allows removal of the ofstream("/dev/null")
   hack Logger used when disabled, which presumably wasn't very portable)

gen/linker.cpp

@@ -333,7 +333,7 @@ int linkObjToExecutable(const char* argv0)
 
     Logger::println("Linking with: ");
     std::vector<const char*>::const_iterator I = args.begin(), E = args.end(); 
-    std::ostream& logstr = Logger::cout();
+    llvm::OStream logstr = Logger::cout();
     for (; I != E; ++I)
         if (*I)
             logstr << "'" << *I << "'" << " ";