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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

The point of this is to prepare for future work where `Kind` can be "scalable offset from CFA" (from the commit message).

> Can you point to an example where the Kind will simplify code?
https://github.com/llvm/llvm-project/pull/168869/files#diff-54d9d06f60ce6c927ea0f2c1380a50bdf93c689d1781186966ef2234660e47c9L406

https://github.com/llvm/llvm-project/pull/168869