Initialize LLVM time trace profiler on each code generation thread

In https://reviews.llvm.org/D71059 LLVM 11, the time trace profiler was
extended to support multiple threads.

`timeTraceProfilerInitialize` creates a thread local profiler instance.
When a thread finishes `timeTraceProfilerFinishThread` moves a thread
local instance into a global collection of instances. Finally when all
codegen work is complete `timeTraceProfilerWrite` writes data from the
current thread local instance and the instances in global collection
of instances.

Previously, the profiler was intialized on a single thread only. Since
this thread performs no code generation on its own, the resulting
profile was empty.

Update LLVM codegen to initialize & finish time trace profiler on each
code generation thread.

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -75,6 +75,10 @@ extern "C" void LLVMTimeTraceProfilerInitialize() {
       /* ProcName */ "rustc");
 }
 
+extern "C" void LLVMTimeTraceProfilerFinishThread() {
+  timeTraceProfilerFinishThread();
+}
+
 extern "C" void LLVMTimeTraceProfilerFinish(const char* FileName) {
   StringRef FN(FileName);
   std::error_code EC;