[clang] [clang-repl] Simplify the logic around out of process execution. NFC (PR #175322)

[Rainer Orth via cfe-commits]

rorth wrote:

Unfortunately this broke the [Solaris/amd64 buildbot](https://lab.llvm.org/staging/#/builders/120/builds/15245) again. I'm quite busy unfortunately, so it will take a while to look into it.

https://github.com/llvm/llvm-project/pull/175322