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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

@rorth, looks like my fix was not quite right. Can you share the output of:
```cpp
./bin/clang -print-resource-dir
./bin/clang -print-runtime-dir
```?

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