RFR: JDK-8315248: AssertionError in Name.compareTo [v2]

[Jonathan Gibbons]

> Please review a small fix to avoid an `AssertionError` coming out of `CharSequence.compare` for two `CharSequence` objects obtained from different instances of the `javax.lang.model` API.
> 
> Full details in the JBS issue and comments.
> 
> There are some minor cleanup edits as well, and some improvements to the code to throw AssertionError in a few places.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  Address review feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15478/files
  - new: https://git.openjdk.org/jdk/pull/15478/files/54f4ea61..f016195f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15478&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15478&range=00-01

  Stats: 11 lines in 2 files changed: 3 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/15478.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15478/head:pull/15478

PR: https://git.openjdk.org/jdk/pull/15478