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.

gen/irstate.cpp

@@ -35,7 +35,7 @@ IRScope &IRScope::operator=(const IRScope &rhs) {
 
 ////////////////////////////////////////////////////////////////////////////////
 IRState::IRState(const char *name, llvm::LLVMContext &context)
-    : module(name, context), DBuilder(this) {
+    : module(name, context), DBuilder(this), objc(module) {
   ir.state = this;
 }