[CFIInserter] Improve `CSRSavedLocation` struct. (#168869)

(1) Define `CSRSavedLocation::Kind` and use it in the code. This makes
the code more readable and allows to extend it to new kinds. For
example, soon I want to add "scalable offset from a given register"
kind.

(2) Store the contents in a union. This should reduce memory usage.

GitOrigin-RevId: e971c3ee0e9205c666cf69fa721226d24e46653f