Fix an assertion error regarding void type with LLVM 3.4.

In Dwarf debug symbol generation the void type was modelled as
DIType(NULL). This triggers an assertion error with LLVM 3.4.
The solution is to use the unspecified type provided by Dwarf for
this purpose.

gen/dibuilder.cpp

@@ -446,7 +446,11 @@ llvm::DIType ldc::DIBuilder::CreateTypeDescription(Type* type,
     }
 
     if (t->ty == Tvoid)
+#if LDC_LLVM_VER >= 304
+        return DBuilder.createUnspecifiedType(t->toChars());
+#else
         return llvm::DIType(NULL);
+#endif
     else if (t->isintegral() || t->isfloating())
     {
         if (t->ty == Tvector)