do not validate variants of concrete specs in solver setup (#22272)

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>

lib/spack/spack/solver/asp.py

@@ -966,7 +966,8 @@ class Body(object):
                 if value == '*':
                     continue
 
-                # validate variant value
+                # validate variant value only if spec not concrete
+                if not spec.concrete:
                     reserved_names = spack.directives.reserved_names
                     if not spec.virtual and vname not in reserved_names:
                         try: