swift234

[driver] Added support for detecting and reporting crashed subprocesses.

Added a TaskSignalledCallback to TaskQueue, which will be called instead of
TaskFinishedCallback if the task exited abnormally.

In Unix/TaskQueue.inc, check WIFSIGNALED if the task did not pass WIFEXITED,
and call the TaskSignalledCallback if necessary. In Default/TaskQueue.inc, check
for a return code of -2; if present, call the TaskSignalledCallback instead of
the TaskFinishedCallback.

Updated Compilation to pass a TaskSignalledCallback.

Added diagnostics to indicate when a command signalled as well as when a command
failed with either poor diagnostics or a non-1 exit code. (These match Clang’s
diagnostics.) Added tests to ensure these diagnostics are emitted when the
frontend crashes or fails an assertion (if assertions are available).

This fixes <rdar://problem/16012199>.

Swift SVN r13654