[SharedCache] Fix handling of relative selectors in macOS shared caches

Find the relative selector base address in the Objective-C optimization
data pointed to by the shared cache header, rather than via
`__objc_scoffs`. This is only present on iOS, and not for every iOS
version that encodes selectors via direct offsets.

This also includes some related improvements:
1. Direct selectors get their own pointer type so they're rendered
   correctly in the view.
2. Method lists encoded as lists of lists are now handled.
3. The `dyld_cache_header` type added to the view is truncated to the
   length in the loaded cache. This ensures it is applied to the view.
4. A couple of methods that process method IMPs and selectors are
   updated to check whether the address is valid before attempting to
   process them. They would otherwise fail by throwing an exception if
   they proceed, but checking for validity is quicker and makes
   exception breakpoints usable.