[clang] [clang-repl] Expose markUserCodeStart() in extended Interpreter interface (PR #87064)

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

Possible, but might add limitations. The init process can be complex. I could imagine a runtime to require actual execution of code (and not only parsing as we'd implement it right now I guess) or load a dynamic library. The patch here allows that, the constructor approach wouldn't.

https://github.com/llvm/llvm-project/pull/87064