Add DyldCache.locateValue for cache hierarchy lookup

[p-x9]

Summary

• Introduce DyldCache.locateValue(_:) (@_spi(Support) public) that resolves a value by walking the receiver → mainCache → main cache's subcaches in order, deduplicating by header UUID.
• Two overloads: a KeyPath<DyldCache, Value?> form for plain optional properties and a rethrows closure form for throwing / computed lookups (e.g. symbolCache).
• Returns a LocatedValue<Value> struct pairing the resolved value with the cache that produced it — needed because offsets inside the value must be interpreted against that cache.

Notes

• @inline(__always) is applied to the thin KeyPath overload and the private _resolve helper so the resolver closure specializes; the closure-form body itself is left as-is since cost is dominated by subcache(for:) I/O.
• Exposed under @_spi(Support) for now to keep flexibility before committing to a fully public API. Symmetric variants on DyldCacheLoaded / FullDyldCache are intentionally deferred.
• Subcache opening can incur file I/O; doc comment recommends caching the resulting LocatedValue when looking up repeatedly.

Test plan

• swift build
• Spot-check via existing DyldCachePrintTests against a local dyld cache (optional, follow-up)