Use checkTyEqRhs to make types concrete

This commit refactors makeTypeConcrete to call checkTyEqRhs with
the appropriate parameters. This avoids duplicating subtle logic
in two places in the compiler.

Changes:

  1. Refactor of 'TyEqFlags'. Now 'TyEqFlags' stores a 'TEFTask', which
     is a description of which of the following checks we want to
     perform in 'checkTyEqRhs':
        - occurs check
        - level check
        - concreteness check

     In the process, the 'AreUnifying' datatype has been removed, as it
     is no longer needed.

  2. Refactor of 'checkTyVar':
      a. Make use of the new 'TEFTask' data type to decide which checks
         to perform.
         In particular, this ensures that we perform **both** a
         concreteness check and a level check when both are required;
         previously we only did a concreteness check (that was a bug!).
      b. Recursively call 'checkTyVar' on the kind of unfilled
         metavariables. This deals with a bug in which we failed to
         uphold the invariant that the kind of a concrete type must
         itself be concrete. See test cases T23051, T23176.

  3. Re-write of 'makeTypeConcrete', which now simply calls
     'checkTyEqRhs' with appropriate 'TyEqFlags'/'TEFTask'.
     This gets rid of code duplication and risk for the two code paths
     going out-of-sync.

Fixes #25616. See also #23883.