Fix and refactor Objective-C state

Tie the state to an LLVM module/object file instead of having a global
one. And finalize it once per LLVM module instead of once per D module
(previously, as part of ModuleInfo generation).

Fixes issue #2388.

driver/codegenerator.cpp

@@ -247,6 +247,8 @@ void CodeGenerator::finishLLModule(Module *m) {
 }
 
 void CodeGenerator::writeAndFreeLLModule(const char *filename) {
+  ir_->objc.finalize();
+
   // Issue #1829: make sure all replaced global variables are replaced
   // everywhere.
   ir_->replaceGlobals();