[llvm] [clang-repl] Support XDG state directory for history (PR #223037)

[via llvm-commits]

RohanOnKeys wrote:

I kept the legacy path as the first choice to preserve existing history and avoid creating a second history file for users who already have ~/.clang-repl-history, which matches the behavior requested in #218266.

https://github.com/llvm/llvm-project/pull/223037