Handle ConcreteTvs in inferResultToType

This patch fixes two issues.

  1. inferResultToType was discarding the ir_frr information, which meant
     some metavariables ended up being MetaTvs instead of ConcreteTvs.

     This function now creates new ConcreteTvs as necessary, instead of
     always creating MetaTvs.

  2. startSolvingByUnification can make some type variables concrete.
     However, it didn't return an updated type, so callers of this
     function, if they don't zonk, might miss this and accidentally
     perform a double update of a metavariable.

     We now return the updated type from this function, which avoids
     this issue.

Fixes #23154

(cherry picked from commit 9ab9b30e)