[clang] Allow for custom code model in clang::Interpreter (PR #156977)

[Jeaye Wilkerson via cfe-commits]

jeaye wrote:

> Is there any reasonable way to test this?

Nothing great came to mind, for testing this in isolation.

One effect it could have is to solve issues like https://github.com/llvm/llvm-project/issues/102858, but that requires exposing this through a `clang-repl` flag and then tackling any other ASan-related issues. I didn't attempt that since it's quite the scope creep.

https://github.com/llvm/llvm-project/pull/156977