[lldb] Teach LLDB about Mach-O filesets

This patch teaches LLDB about Mach-O filesets. Filsets are Mach-O files
that contain a bunch of other Mach-O files. Unlike universal binaries,
which have a different header, Filesets use load commands to describe
the different entries it contains.

Differential revision: https://reviews.llvm.org/D132433