swift234

Only enable CrashTracer support when explicitly specified.

This will (by default) disable CrashTracer being linked in for
developer builds of Swift.  By default, our build presets
will only enable CrashTracer support for B&I builds.

Implements:

<rdar://problem/18177298> Only enable CrashTracer support for Swift compiler in B&I

Swift SVN r27661