Improve unique_ptr-y ownership in ASTUnit::ComputePreamble

Rather than having a pair of pairs and a reference out parameter, build
a structure with everything together and named. A raw pointer and a
unique_ptr, rather than a raw pointer and a boolean, are used to
communicate ownership transfer.

It's possible one day we'll end up with a conditional pointer (probably
represented by a raw pointer and a boolean) abstraction to use in places
like this. Conditional ownership seems to be coming up more often than
I'd hoped...