With this CL, we set the is_source_positions_enabled flag on CompilationInfo when
- a command line flag is enabled that requires Turbofan to preserve source position
  information (e.g. --trace-deopt), and
- when profiling is enabled.

This also removes the --turbo-source-positions flag.

The goal is to eventually only track source position information when needed.

R=mstarzinger@chromium.org
BUG=v8:5439

Review-Url: https://codereview.chromium.org/2484163003
Cr-Commit-Position: refs/heads/master@{#40836}