RFR: 8258810: Improve enum traits [v2]

[Kim Barrett]

> Please review these miscellaneous improvements to the enum iteration facility.
> Thanks to @albertnetymk for discussions.
> 
> (1) Removed some unneeded stuff from EnumIterationTraits.
> 
> (2) Various similar bits of error checking code are unified and shared via
> EnumIterationTraits.
> 
> (3) Casts between the enum type and the underlying type are now done using
> helper functions in EnumIterationTraits.
> 
> (4) JDK-8257466 added accessors for the EnumRange bounds and the index for
> an enumerator, but didn't account for the possibility of an empty subrange.
> Add assertions for that case.
> 
> (5) Added some death tests to verify some of the error checking.
> 
> Testing:
> mach5 tier1

Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:

  ayang review

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1868/files
  - new: https://git.openjdk.java.net/jdk/pull/1868/files/cb889e87..a4667b70

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1868&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1868&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1868.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1868/head:pull/1868

PR: https://git.openjdk.java.net/jdk/pull/1868