Introduce an extension method to retrieve the set of applicable targets
for an annotation, given the annotation class's symbol.
This extension on `KtSymbolInfoProvider` causes a large amount of churn
in test data files, so this change starts off by introducing the
property and always making it return null. A future change will
introduce the actual behavior change, and will include the testdata
changes for the few tests that actually return useful data for this
property.
Review: https://jetbrains.team/p/kt/reviews/7652
Put this change into a separate commit because:
1. The change is boring and clutters the diff of the first commit
2. The first commits is already too big
Dmitrii Gridin