RFR: 8290041: ModuleDescriptor.hashCode is inconsistent [v2]

[Jaikiran Pai]

> Can I please get a review of this change which proposes to fix https://bugs.openjdk.org/browse/JDK-8290041? 
> 
> As noted by the reporter, the current implementation is buggy since the calculation can result in a different value of the hashcode depending on the order of iteration of the `Modifier`s. The commit in this PR changes that computation to produce consistent result irrespective of the order in which the `Modifier`s (enum) is iterated upon.
> 
> A new test has been added which reproduces the issue and verifies the fix.

Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:

  incorporate review comments and add more testing

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9790/files
  - new: https://git.openjdk.org/jdk/pull/9790/files/13af6b83..4933e6d6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9790&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9790&range=00-01

  Stats: 98 lines in 2 files changed: 85 ins; 6 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/9790.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9790/head:pull/9790

PR: https://git.openjdk.org/jdk/pull/9790