Select messages should be ordered by declaration ID

[dschrempf]

At the moment we emit trace messages in the select pass in a pretty randomly-chosen order:

    let ...
    in  (    unitSelect
        ,    selectStatusMsgs
          -- If there were no predicate matches we issue a warning to the user.
          ++ [ SelectNoDeclarationsMatched | Set.null rootIds ]
          ++ getDelayedMsgs      selectedIndex

Ideally we sort the messages, for example by declaration ID.

[dschrempf]

Thinking about declaration order and indexing:

• The list of declarations attached to the translation unit determines the order
  of declarations. This order is determined in the ConstructTranslationUnit
  pass (UseDeclGraph.fromDecls). I guess this mostly coincides with the order
  in the C source code.
• We do not store the order of declarations by any other means.
• We also do not give indices to our declarations (we use QualPrelimDeclId to
  identify declarations, mostly).

Now, with respect to sorting messages in the Select pass:

• We could omit the delayed parse messages next to the select messages (which
  are already sorted), and all other delayed parse messages at the end (ordered
  by QualPrelimDeclId). I believe this is a quick solution (but it does not
  assign proper indices to the declarations).
• We could attach proper indices to our declarations after sorting them in
  ConstructTranslationUnit. This is a bit complicated, because these indices
  should be in the DeclIndex, but when we construct the DeclIndex, we still
  don't know the order (chicken-egg problem). We could also assemble a
  "bijective map" QualPrelimDeclId <-> Index, and consult this map when
  sorting the messages.

[dschrempf]

We do not "sort" failed declarations, so we instead decided to simply sort by source location, and use the include graph to figure out the order between headers.