[llvm-profgen] Harden perf script invocation (#212253)
Harden ``llvm-profgen``'s ``perf script`` invocation for ``--perfdata``:
report launch/exit failures, and clear redirect files between the two
invocations so stale stdout/stderr cannot leak. Cover both with lit
tests that inject a mock ``perf`` via ``PATH``.
Assisted by GPT-5
