[clang] [REAPPLY][Clang-Repl] Add support for out-of-process execution. #110418 (PR #144064)

[Teresa Johnson via cfe-commits]

teresajohnson wrote:

I just tracked down slowness running clang tests locally to this change. It's very noticeable if I run a single test, or a single directory of tests.

When I ctrl-c the stack trace includes:
```
    clang_repl_cmd = subprocess.run(
        [clang_repl_exe, "-orc-runtime", "-oop-executor"],
```

I ran a single clang test 5 times each with current HEAD and with a "return False" right at the start of have_host_out_of_process_jit_feature_support.

Here are the user time comparisons:

```
time.out.off:	User time (seconds): 0.38
time.out.off:	User time (seconds): 0.36
time.out.off:	User time (seconds): 0.34
time.out.off:	User time (seconds): 0.40
time.out.off:	User time (seconds): 0.36
time.out.on:	User time (seconds): 17.80
time.out.on:	User time (seconds): 17.83
time.out.on:	User time (seconds): 17.80
time.out.on:	User time (seconds): 17.64
time.out.on:	User time (seconds): 17.71
```

Can something be done to prevent this overhead?

https://github.com/llvm/llvm-project/pull/144064