swift234

[driver] Implemented support for emitting parseable output.

When "-parseable-output" is passed to the driver, it will now emit output in a
parseable format. (This format is described in docs/DriverParseableOutput.rst,
which was added in a previous commit.)

This is achieved by adding four functions (one for each kind of message). These
are in a new swift::driver::parseable_output namespace, and given the right
parameters, will output the appropriate message in JSON to the given
llvm::raw_ostream. These functions are then called by
Compilation::performJobsInList:

  - "began" messages are emitted by the taskBegan callback
  - "finished" messages are emitted by the taskFinished callback
  - "signalled" messages are emitted by the taskSignalled callback
  - "skipped" messages are emitted by the handleCommandWhichDoesNotNeedToExecute
    lambda

(Note that "skipped" messages will not be emitted in practice, since the driver
does not yet support partial compilation.)

This fixes <rdar://problem/15958329>.

Swift SVN r20873