RFR: 8286849: Use @Stable for generic repositories

[liach]

Generic repositories, the implementation detail for generic information in core reflection, can be updated to use the `@Stable` annotation to replace their `volatile` access. Their existing accessor code is already safe, reading the cache fields only once.

In addition, fixed potentially non-thread-safe `genericInfo` access in `Method`, `Field`, and `RecordComponent`.

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Commit messages:
 - 8286849: Use @Stable for generic repositories

Changes: https://git.openjdk.java.net/jdk/pull/8742/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8742&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286849
  Stats: 45 lines in 11 files changed: 29 ins; 0 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8742.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8742/head:pull/8742

PR: https://git.openjdk.java.net/jdk/pull/8742