Introduce a debug info builder.

Moves all code from todebug into a new class.
Also caches the compilation unit in order to fix a LLVM 3.4 compile error.

gen/irstate.cpp

@@ -68,7 +68,7 @@ IRTargetScope::IRTargetScope(
 
 //////////////////////////////////////////////////////////////////////////////////////////
 IRState::IRState(llvm::Module* m)
-    : module(m), dibuilder(*m)
+    : module(m), DBuilder(this, *m)
 {
     interfaceInfoType = NULL;
     mutexType = NULL;