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/librustc_codegen_llvm/lib.rs

@@ -332,6 +332,12 @@ impl CodegenBackend for LlvmCodegenBackend {
         // any more, we can finalize it (which involves renaming it)
         rustc_incremental::finalize_session_directory(sess, codegen_results.crate_hash);
 
+        sess.time("llvm_dump_timing_file", || {
+            if sess.opts.debugging_opts.llvm_time_trace {
+                llvm_util::time_trace_profiler_finish("llvm_timings.json");
+            }
+        });
+
         Ok(())
     }
 }