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

[Rainer Orth via cfe-commits]

rorth wrote:

I cannot check the actual bot since it just started a new run, but in another `projects` build it's in `clang/21/lib/sunos/liborc_rt-x86_64.a`.  There **must** be some way to locate runtime libs without every command that needs that second-guessing...

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