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/dwarftypes.cpp

@@ -16,7 +16,6 @@
 #include <llvm/Analysis/Verifier.h>
 #include <llvm/Assembly/PrintModulePass.h>
 #include <algorithm>
-#include <iostream>
 
 void RegisterDwarfSymbols(llvm::Module* mod) {
   using namespace llvm;