RFR: 8284871: Use covariant overrides for the resolveConstantDesc(Lookup) method in sub‑interfaces of java.lang.constant.ConstantDesc

[liach]

Also remove redundant casts to avoid javac warnings.

-------------

Commit messages:
 - Move MethodTypeDesc api notes to MethodTypeDesc
 - Merge branch 'master' of https://git.openjdk.java.net/jdk into constdesc-override
 - Remove redundant casts
 - Covariant override of resolveConstantDesc for Class/MethodType/MethodHandleDesc

Changes: https://git.openjdk.org/jdk/pull/12986/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12986&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8284871
  Stats: 28 lines in 8 files changed: 18 ins; 4 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/12986.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12986/head:pull/12986

PR: https://git.openjdk.org/jdk/pull/12986