[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

bolt/lib/Profile/DataAggregator.cpp

@@ -216,17 +216,7 @@ void DataAggregator::launchPerfProcess(StringRef Name, PerfProcessInfo &PPI,
   outs() << "PERF2BOLT: spawning perf job to read " << Name << '\n';
   Argv.push_back(PerfPath.data());
 
-  char *WritableArgsString = strdup(ArgsString);
-  char *Str = WritableArgsString;
-  do {
-    Argv.push_back(Str);
-    while (*Str && *Str != ' ')
-      ++Str;
-    if (!*Str)
-      break;
-    *Str++ = 0;
-  } while (true);
-
+  StringRef(ArgsString).split(Argv, ' ');
   Argv.push_back("-f");
   Argv.push_back("-i");
   Argv.push_back(Filename.c_str());
@@ -266,8 +256,6 @@ void DataAggregator::launchPerfProcess(StringRef Name, PerfProcessInfo &PPI,
   else
     PPI.PI = sys::ExecuteNoWait(PerfPath.data(), Argv, /*envp*/ std::nullopt,
                                 Redirects);
-
-  free(WritableArgsString);
 }
 
 void DataAggregator::processFileBuildID(StringRef FileBuildID) {