[SharedCache] Use basic copy-on-write for viewStateCache

Copying the state from the cache into a new `SharedCache` object is done
with a global lock held and is so expensive that it results in much of
the shared cache analysis running on a single thread, with others
blocked waiting to acquire the lock.

The cache now holds a `std::shared_ptr` to the state. New `SharedCache`
objects take a reference to the cached state and only create their own
copy of it the first time they perform an operation that would mutate
it. The cached copy is never mutated, only replaced, so there is no
danger of modifying the state out from under a `SharedCache` object.
Since the copy happens at first mutation, it is performed without any
global locks held. This avoids blocking other threads.

This cuts the initial load time of a macOS shared cache from 3 minutes
to 70 seconds, and cuts the time taken to load and analyze AppKit from
multiple hours to around 14 minutes.

Author: bdash