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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> This seems to have broken the [Solaris/amd64 buildbot](https://lab.llvm.org/staging/#/builders/120/builds/15163).

Apologies, will provide a fix shortly. 

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