[DSC] Rework handling of .symbols files to be compatible with iOS 15

In some iOS 15 caches, the .symbols file's mapping has an address of 0.
This would cause it to be returned by `SharedCache::GetEntryContaining`
and loaded into the view.

The .symbols file contains the local symbol tables for images in the
shared cache. It is not intended to be mapped into the same address
space as the rest of the shared cache.

`SharedCache` now tracks the symbols cache entry separately from other
entries. A dedicated `VirtualMemory` region is used when accessing the
data it contains. This could be a `FileAccessor`, but that would require
additional changes within `SharedCacheMachOHeader`.

`SharedCacheMachOProcessor` now directly accesses the local symbols
cache entry rather than needing to search for it.