[clang] [clang-repl] Factor out CreateJITBuilder() and allow specialization in derived classes (PR #84461)

[Stefan Gränitz via cfe-commits]

https://github.com/weliveindetail commented:

This PR depends on two predecessor patches, but I wanted to share it for review already. For the moment please refer to the single commit in https://github.com/llvm/llvm-project/pull/84461/commits/cc46034e5288ba54dfc8a0ea7d54792cbb99227d for review.

https://github.com/llvm/llvm-project/pull/84461