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.
Jinseong Jeon