[clang] [Clang-Repl] Adds custom lambda in launchExecutor and PID retrieval (PR #152562)

[Rainer Orth via cfe-commits]

rorth wrote:

The new check is certainly better since it explicitly excludes anything but macOS and Linux/**x86_64** which are the only ones that are supported right now.  However, the code still doesn't account for the two possible layouts of the runtime dir.  Rather than hardcoding those things, it would be way better to just determine them at runtime just like `clang --print-runtime-dir` does.

https://github.com/llvm/llvm-project/pull/152562