Currently, regardless of a spec being concrete or not, we validate its variants in `spec_clauses` (part of `SpackSolverSetup`).
This PR skips the check if the spec is concrete.
The reason we want to do this is so that the solver setup class (really, `spec_clauses`) can be used for cases when we just want the logic statements / facts (is that what they are called?) and we don't need to re-validate an already concrete spec. We can't change existing concrete specs, and we have to be able to handle them *even if they violate constraints in the current spack*. This happens in practice if we are doing the validation for a spec produced by a different spack install.
Signed-off-by: vsoch <vsoch@users.noreply.github.com>
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