RFR: 8286057: Make javac error on a generic enum friendlier [v3]

[Aggelos Biboudis]

> Improves the error message for generic enumerations.

Aggelos Biboudis has updated the pull request incrementally with one additional commit since the last revision:

  Add precise error for enums with empty parameter lists

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8677/files
  - new: https://git.openjdk.java.net/jdk/pull/8677/files/0d7db485..401b9754

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8677&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8677&range=01-02

  Stats: 35 lines in 3 files changed: 33 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8677.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8677/head:pull/8677

PR: https://git.openjdk.java.net/jdk/pull/8677