Mostly rewrite debug info generation in terms of llvm/Analysis/DebugInfo.h.

Add getCompilationModule to Dsymbol and fix template compile unit decision code.
Runtime compiles with -g again.

ir/irmodule.cpp

@@ -10,8 +10,6 @@ IrModule::IrModule(Module* module, const char* srcfilename)
     LLConstant* slice = DtoConstString(srcfilename);
     fileName = new llvm::GlobalVariable(
         slice->getType(), true, LLGlobalValue::InternalLinkage, slice, ".modulefilename", gIR->module);
-
-    dwarfCompileUnit = NULL;
 }
 
 IrModule::~IrModule()