Changed the way swift-backtrace works for JSON, so that it captures
into a CrashLog, then writes to JSON from the CrashLog. Separating out
the two sets of logic so that writing the CrashLog to JSON becomes
simpler and more modular.
It's useful to capture the platform and platform version with the image map.
Also, display both the platform and architecture information when generating
a crash log.
rdar://124913332
If we ended up with a `/` at the beginning of a string segment, we
were erroneously not adding to the expansion dictionary when we
should have been.
rdar://124913332
We want to be able to efficiently serialise lists of images, and to do so
it makes most sense to create a separate `ImageMap` type. This also provides
a useful place to put methods to e.g. find an image by address or by build
ID.
rdar://124913332
