[TimeProfiler] Reset variable to nullptr

[smeenai]

Summary

Otherwise we'll hit a spurious assert failure when we reset and then
reinitialize TimeProfiler on the same thread, as can happen when e.g.
using LLD as a library and running it multiple times in the same
process.

Makes lld/test/MachO/time-trace.s pass with LLD_IN_TEST=2, which
runs the linker twice in the same process and exposed the issue.

[mehdi_amini]

LGTM

llvm/lib/Support/TimeProfiler.cpp

void llvm::timeTraceProfilerInitialize(unsigned TimeTraceGranularity,
   TimeTraceProfilerInstance = new TimeTraceProfiler(
       TimeTraceGranularity, llvm::sys::path::filename(ProcName));
 }
 
 // Removes all TimeTraceProfilerInstances.
 // Called from main thread.
 void llvm::timeTraceProfilerCleanup() {
   delete TimeTraceProfilerInstance;
+  TimeTraceProfilerInstance = nullptr;
   std::lock_guard<std::mutex> Lock(Mu);
   for (auto *TTP : *ThreadTimeTraceProfilerInstances)
     delete TTP;
   ThreadTimeTraceProfilerInstances->clear();
 }
 
 // Finish TimeTraceProfilerInstance on a worker thread.
 // This doesn't remove the instance, just moves the pointer to global vector.