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

[Rainer Orth via cfe-commits]

rorth wrote:

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

lib/clang/22

> ./bin/clang -print-runtime-dir

lib/clang/22/lib/sunos
```
Please note that the Solaris bots still use a project build to avoid the issue now finally fixed in PR #166837.  This seems to have reached conclusion now (provided it won't be backed out again), so I can probably switch the bots to runtimes builds.  However, that will require approval first.


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