Emit pre-splicing references when using TH

[robinp]

Now $(foo) is indexed with the unspliced content, but the reference to foo itself doesn't get emitted. This causes for example that backreferences on foo don't show the usage sites.

Related:

• Q expressions bar = [|$(foo)|].
• QuasiQuoter name [bar|1+1].

Can we access the pre-splicing info from some of the ASTs?

[mpickering]

It should also be possible to index the post processed sources eventually as well?

[robinp]

@mpickering: forked to #58 - but the two issues are very intertwined. I feel some more investigation / brainstorming is needed before making up a plan.