swift234

Add the frontend flag '-external-pass-pipeline-filename'

This flag enables one to specify a json file that expresses a specific
pipeline in the following format:

[
  [
    "$PASS_MANAGER_ID",
    "run_n_times"|"run_to_fixed_point",
    $NUM_ITERATIONS,
    "$PASS1", "$PASS2", ...
  ],
  ...
]

This will make it easier to experiment with different pass pipelines by
allowing:

1. Automatic generation of pass pipelines without needing to recompile
   the compiler itself.
2. Simple scripting of pass pipelines via the json meta language.
3. Enabling the easy expression and reproducability of a specific
   pipeline ordering via radar.

In the next commit I will provide a python library for the generation of these
json files with a few types of pipeline generators already created.

Swift SVN r24055