LLVM time profiler can collect time traces and present them in a
hierarchical view. Which breakdowns the time spent in each Pass or even
IR unit. The result is also exported into a format that can be easily
visualized by the Chrome browser.

Currently this features is controlled by the following environment
variables:
 - `ZIG_LLVM_TIME_TRACE_FILE` toggles this feature and specifies the
   output time trace file.
 - `ZIG_LLVM_TIME_TRACE_GRANULARITY` controls the time granularity in
   ms (default to 500).