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/llvmhelpers.cpp

@@ -546,7 +546,7 @@ DValue* DtoNullValue(Type* type)
     }
 
     // unknown
-    std::cout << "unsupported: null value for " << type->toChars() << '\n';
+    llvm::cout << "unsupported: null value for " << type->toChars() << '\n';
     assert(0);
     return 0;