[clang] [clang-repl] Expose RuntimeInterfaceBuilder to allow customization (PR #83126)

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

This is an early proposal. I'd like to collect feedback before polishing. Let's focus on the conceptual questions. I tried to keep the patch minimal, so that it's easier to review. Each of the three commits depends on the previous ones. They should work and be reviewed in isolation (I guess). What do you think?

https://github.com/llvm/llvm-project/pull/83126