[BOLT][NFCI] Use StringRef.split in launchPerfProcess

Use StringRef method instead of reimplementing the splitting.
Incidentally, it also fixes the duplicate printing of the command arguments:
```
PERF2BOLT: spawning perf job to read branch events
Launching perf: /usr/bin/perf script^@-F^@pid,ip,brstack -F^@pid,ip,brstack pid,ip,brstack -f -i
PERF2BOLT: spawning perf job to read mem events
Launching perf: /usr/bin/perf script^@-F^@pid,event,addr,ip -F^@pid,event,addr,ip pid,event,addr,ip -f -i
PERF2BOLT: spawning perf job to read process events
Launching perf: /usr/bin/perf script^@--show-mmap-events --show-mmap-events -f -i
PERF2BOLT: spawning perf job to read task events
Launching perf: /usr/bin/perf script^@--show-task-events --show-task-events -f -i
```

Fixes it to:
```
PERF2BOLT: spawning perf job to read branch events
Launching perf: /usr/bin/perf script -F pid,ip,brstack -f -i
PERF2BOLT: spawning perf job to read mem events
Launching perf: /usr/bin/perf script -F pid,event,addr,ip -f -i
PERF2BOLT: spawning perf job to read process events
Launching perf: /usr/bin/perf script --show-mmap-events -f -i
PERF2BOLT: spawning perf job to read task events
Launching perf: /usr/bin/perf script --show-task-events -f -i
```

Reviewed By: #bolt, rafauler

Differential Revision: https://reviews.llvm.org/D152483