[AA] Add KtEnumEntryInitializerSymbol

- An enum entry's body is an initializer with members only accessible
  within that body. Because users of the Analysis API might want to
  analyze the members of the enum entry initializer, we expose this
  initializer via `KtEnumEntrySymbol`. The initializer only exists if
  the enum entry has a body.
  - We already have some usages of the initializer inside symbol light
    classes, which generate a light class for each enum entry, which
    includes the enum entry's hidden members.
- To hide the implementation detail that initializers are anonymous
  objects, `KtEnumEntryInitializerSymbol` is simply a
  `KtSymbolWithMembers`.
- The advantage of making it a `KtSymbolWithMembers`, instead of
  providing a custom way to get a member scope, is that we can pass
  around the initializer easily, e.g. in `KtEnumEntrySymbolRenderer`.
- We implement `KtEnumEntryInitializerSymbol` directly as a
  `KtFirAnonymousObjectSymbol` without a wrapper. This has a few
  advantages:
  1. We can directly benefit from the anonymous object symbol being a
     `KtSymbolWithMembers`, so we don't have to handle enum entry
     initializers specially in e.g. `KtFirScopeProvider`.
  2. We don't have to implement a new symbol restoration mechanism for
     the initializer.
  3. This implementation matches the actual FIR tree structure (with a
     simplification that the connecting anonymous object expression
     between the enum entry and the initializing anonymous object is
     omitted).

^KT-61425 fixed

analysis/analysis-api/testData/components/scopeProvider/scopeContextForPosition/enumEntry.txt

@@ -519,6 +519,7 @@ scopes:
         callableIdIfNonLocal: /E.A
         containingEnumClassIdIfNonLocal: E
         contextReceivers: []
+        enumEntryInitializer: KtAnonymousObjectSymbol(<local>/<no name provided>)
         isExtension: false
         name: A
         origin: SOURCE