[SharedCache] Make it faster to look up which image contains an address

`MemoryRegion` now tracks the start address of the image to which it
belongs. `SharedCache::HeaderForAddress` uses the existing address range
map to quickly find the `MemoryRegion` for a given address, and from
there can look up the image via its start address.

Some extra logic is added to `CacheInfo::Load` to populate the image
start address on `MemoryRegion` when loading from metadata that predates
this change.

The result is that `HeaderForAddress` is no longer the most expensive
part of `FindSymbolAtAddrAndApplyToAddr`.