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
