Improved error messages from clingo (#26719)

This PR adds error message sentinels to the clingo solve, attached to each of the rules that could fail a solve. The unsat core is then restricted to these messages, which makes the minimization problem tractable. Errors that can only be generated by a bug in the logic program or generating code are prefaced with "Internal error" to make clear to users that something has gone wrong on the Spack side of things.

* minimize unsat cores manually

* only errors messages are choices/assumptions for performance

* pre-check for unreachable nodes

* update tests for new error message

* make clingo concretization errors show up in cdash reports fully

* clingo: make import of clingo.ast parsing routines robust to clingo version

Older `clingo` has `parse_string`; newer `clingo` has `parse_files`.  Make the
code work wtih both.

* make AST access functions backward-compatible with clingo 5.4.0

Clingo AST API has changed since 5.4.0; make some functions to help us
handle both versions of the AST.

Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

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