Based on feedback in PR https://github.com/swiftlang/swift/pull/69460, enabling indexing for synthesized decls because they are usable by users and make sense to appear in the indexstore.
Sets `synthesized` on some additional decls:
- derived `hashInto(...)`
- Objc properties and methods derived from Objc protocols
https://github.com/apple/swift/issues/67446
Check to see whether we can index the given decl
before reporting it as a container, walking up to
a parent if we need to. This also lets us simplify
the AnyPattern handling a bit.
rdar://126137541
References associated with a `VarDecl` had no `RelationContainedBy` role, resulting in "orphaned" references. From the perspective of identifying unused code (in tools using the index, like [Periphery](https://github.com/peripheryapp/periphery)), this made it impossible to identify that a variable's type, initializer and custom attributes are associated with the variable.
Resolves: [SR-13766](https://bugs.swift.org/browse/SR-13766)
