Fix `run_clang_repl` output when not present (#161412)
On the happy path, when `clang-repl` is present, we will invoke it in
order to determine if the host supports JIT features. That will return a
string containing "true". However, in cases where `clang-repl` is not
present or we fail to invoke it, we previously returned `False`, which
would then trigger a failure with our substring check. This PR updates
the function to return `""` instead, so the substring check is still
valid.
This is related to https://github.com/llvm/llvm-project/pull/157359,
where the original change was introduced.
