[mach-o] propagate dylib version numbers

Mach-o does not use a simple SO_NEEDED to track dependent dylibs.  Instead,
the linker copies four things from each dylib to each client: the runtime path
(aka "install name"), the build time, current version (dylib build number), and
compatibility version  The build time is no longer used (it cause every rebuild
of a dylib to be different).  The compatibility version is usually just 1.0
and never changes, or the dylib becomes incompatible.

This patch copies that information into the NormalizedMachO format and
propagates it to clients.

llvm-svn: 222300