Add support for enabling the LLVM time-trace feature

I found this helpful while investigating an LLVM performance issue.
Passing `-Z llvm-time-trace` causes a `llvm_timings.json` file to be
created. This file can be inspected in either the Chrome Profiler tools
or with any other compatible tool like SpeedScope.

More information on the LLVM feature:

- https://aras-p.info/blog/2019/01/16/time-trace-timeline-flame-chart-profiler-for-Clang/

- https://reviews.llvm.org/rL357340

src/rustllvm/PassWrapper.cpp

@@ -26,6 +26,7 @@
 #include "llvm/Transforms/Instrumentation.h"
 #if LLVM_VERSION_GE(9, 0)
 #include "llvm/Transforms/Instrumentation/AddressSanitizer.h"
+#include "llvm/Support/TimeProfiler.h"
 #endif
 #if LLVM_VERSION_GE(8, 0)
 #include "llvm/Transforms/Instrumentation/ThreadSanitizer.h"
@@ -57,6 +58,23 @@ extern "C" void LLVMInitializePasses() {
   initializeTarget(Registry);
 }
 
+extern "C" void LLVMTimeTraceProfilerInitialize() {
+#if LLVM_VERSION_GE(9, 0)
+  timeTraceProfilerInitialize();
+#endif
+}
+
+extern "C" void LLVMTimeTraceProfilerFinish(const char* FileName) {
+#if LLVM_VERSION_GE(9, 0)
+  StringRef FN(FileName);
+  std::error_code EC;
+  raw_fd_ostream OS(FN, EC, sys::fs::CD_CreateAlways);
+
+  timeTraceProfilerWrite(OS);
+  timeTraceProfilerCleanup();
+#endif
+}
+
 enum class LLVMRustPassKind {
   Other,
   Function,